Zesty key lime cake baked right onto a graham cracker crust, and slathered with key lime buttercream and graham cracker crumbles, this cake is everything I love about a key lime pie… but it’s a cake! We’re continuing my sheet cake series with cake #7 (three more to go!) – today we have my key lime pie sheet cake with graham cracker crust.

I know this recipe is a favorite of yours! Originally shared back in 2019 in my Cake Confidence cookbook, my Key Lime Pie Cake is packed full of gorgeous key lime flavor, a graham cracker crust, and the most fluffy key lime buttercream! When I asked my Instagram followers what cakes I they’d like to see turned into a sheet cake, this one by far got the most votes. I cannot get enough of this cake, and I also know it’s going to be love at first bite if you haven’t tried it yet!

PS – if you’re into key lime, I have an AMAZING Key Lime Pie that we make every single year for Thanksgiving.

Ingredients for Key Lime Pie Sheet Cake

This cake is based off a doctored cake mix, so that means you’ll be using a cake mix BUT also adding in a bunch of ingredients to make this cake taste better than scratch. It’s fool proof, fluffy, delicious, and with that crust on the bottom, it’s going to be an instant hit in your kitchen!

Key Lime Cake Batter

  • Buttermilk – room temperature. If you can’t find it, you can make with whole milk and lemon juice or vinegar to curdle it beforehand.
  • Sour Cream – you could also use plain or key lime greek yogurt.
  • Egg Whites – large eggs, room temperature
  • Vanilla – pure vanilla
  • Vegetable Oil
  • Key Lime Zest – yes, they taste different than regular limes, so it’s worth it if you can find the key limes.
  • Key Lime Juice – you can squeeze 20 key limes or just buy the key lime juice. 🙂
  • White Cake Mix – I use Duncan Hines brand, but anything else should be similar.

Graham Cracker Crust:

  • Graham cracker powder – made by pulsing whole graham crackers into a powder, THEN measuring out the three cups that are needed.
  • Butter – melted, unsalted.

The recipe is below – you’ll only need a bowl and whisk to combine the ingredients, so no need for a stand mixer. Save that for your buttercream!

Do I have to make this into a sheet cake?

Nope! You can make it into a bundt cake (omitting the crust, obviously), cupcakes, two 8″ cake rounds, three 6″ rounds, or one large 10″ cake round if you’d like. The only thing that changes is the baking time.

Here’s a link to the sheet pan I used.

How to make key lime buttercream

The buttercream is pretty easy! Just combine the ingredients with a stand mixer, then flip the switch to high speed so the buttercream can whip up to a lighter texture before spreading it onto your sheet cake. Here’s what you’ll need:

  • Unsalted Butter – leave it out at room temperature for 15 minutes so it’s not super hard to begin with. It will combine & whip up better when it has had time to sit out first.
  • Salt – it cuts through the sweet. Just a pinch will do.
  • Vanilla – pure vanilla
  • Heavy Cream – I love the 40% Darigold brand.
  • Key Lime Juice – Use freshly squeezed if possible. I know Trader Joes sells key lime juice, too.
  • Key Lime Zest – I know it’s tedious, but I promise this is where you’ll get the best key lime flavor.
  • Powdered Sugar – no need to sift, but keep the mixer speed on low and add it in a little at a time.

The directions for the buttercream are below with the recipe instructions.

Can I use regular limes? Does it have to be key limes?

Yes and no. Key limes have a distinct, more floral, flavor. It’s more soft and sweet. I know they can be tricky to find sometimes, depending on the season and what store you’re visiting. If you can’t find them, I know there’s store-bought key lime juice that will help. You may also use regular limes – it won’t taste exactly like key limes, but I’ve made it both ways and they were both great! However, you need the tiny key lime slices for this darling decoration, so it’s up to you!

Is there an easier way to frost this sheet cake?

Of course! Just slather on the buttercream, omit the key lime slices, and sprinkle on the graham cracker and key lime zest on top. Easy peasy!

What piping tip is that?

I used the large Ateco 869 piping tip. I love this open french piping tip – the small groves make such a cool pattern, and it’s easy to make decorative dollops between the key lime slices!

Here’s a video of the decorating process:

As always, I LOVE to see your bakes and cakes inspired by a recipe or design I’ve shared. Make sure to tag me @bakingwithblondie or #bakingwithblondie on social media so I can join in on all the fun.

Happy Baking!

xo, Mandy

Get the Recipe: Key Lime Pie Sheet Cake with Graham Cracker Buttercream

Soft key lime cake layers baked onto a graham cracker crust, frosted in decadent key lime buttercream, topped with a graham cracker crumble, and key lime slices.
Servings: 20 slices, depending on how you slice it
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Ingredients
 

Key Lime Pie Cake with Graham Cracker Crust

  • 3 cups crushed graham cracker powder , (graham crackers pulsed into a powder, then 3 cups measured from it)
  • 1 stick (or 1/2 cup) melted unsalted butter
  • 3/4 cup buttermilk , room temperature
  • 2/3 cup sour cream, room temperature
  • 4 egg whites, room temperature
  • 1/3 cup vegetable oil
  • 1 Tablespoon vanilla
  • zest from 2 limes (about 2 teaspoons)
  • 1/4 cup lime juice (squeeze from key limes or fresh limes)
  • 1 15.25oz Box White Cake Mix, I prefer the taste and texture of Duncan Hines cake mixes, but anything similar should be fine.

Key Lime Buttercream

  • 1 1/2 cup (or 3 sticks) unsalted butter, left out at room temperature for 15 minutes
  • 1 pinch salt
  • 1 Tablespoon vanilla
  • 2-3 teaspoons fresh key lime zest
  • 1/8 cup key lime juice
  • 1/4 cup heavy cream
  • 7-8 cups powdered sugar

Extras/Garnish

  • 1 cup crumbled graham crackers
  • 1 cup sliced key limes for garnish
  • 2 Tablespoons key lime zest

Equipment

  • Large French Star Tip
  • food processor
  • Citrus Zester
  • 9×13 pan

Instructions
 

For the Cake

  • Preheat the oven to 325 degrees. Prep a 9×13 sheet cake pan or dish with a swipe of shortening and dust of flour (or use nonstick baking spray) to prevent sticking. Set aside.
  • Make the crust by pulsing 3 packages (about 9 graham crackers each package), then measuring out three cups of graham cracker powder. Stir in the melted butter until thoroughly combined. Press evenly into the prepared sheet cake pan and press firmly with the bottom of a glass cup or flat measuring cup. Set aside – no prebaking required.
  • In a large bowl, hand whisk together all the ingredients except for the white cake mix. Sift in the cake mix, then gently stir to combine. Pour the cake batter onto the crust, then bake for 27-30 minutes until the center is baked through. The cake should not be golden on top – don't overbake! Let cool in the pan completely before adding on the frosting.

For the Key Lime Frosting

  • In the bowl of a stand mixer fitted with a paddle attachment, whip up the butter until it's light and fluffy. Add in the salt, vanilla, zest, lime juice, and heavy cream.
  • Scrape down the sides of the bowl, then turn the mixer to the lowest speed. Slowly add in the powdered sugar, then after it's all been added, flip the mixer onto the highest speed until the texture is light and fluffy. Add in more heavy cream (a tablespoon at a time between mixing) to thin the buttercream if needed. Use a wooden spoon to hand stir out any air bubbles.

Assembly

  • Spread out the buttercream flat with an offset icing spatula or cake scraper. With the large french star tip, pipe dollops around the outside rim of the cake; carefully placing a little key lime wedge between the dollops.
  • Sprinkle the graham cracker crumbles on the inside of the dollops, then sprinkle on the key lime zest everywhere. Enjoy!
  • *Alternative simple method: Spread on the buttercream in an even layer over the top of the cake, sprinkle on the graham cracker crumbs and key lime zest on top.
Author: Mandy – Baking with Blondie