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Key Lime Pie Sheet Cake with Graham Cracker Buttercream

Print Recipe
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
Author Mandy - Baking with Blondie

Equipment

  • Large French Star Tip
  • food processor
  • Citrus Zester
  • 9x13 pan

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

Instructions

For the Cake

  • Preheat the oven to 325 degrees. Prep a 9x13 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.