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Lemon Blueberry Sheet Cake with Lemon Curd

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The most gorgeous lemon blueberry sheet cake slathered with lemon curd and topped with silky blueberry buttercream.
Servings 12 slices
Author Mandy Merriman - Baking with Blondie

Ingredients

Lemon Blueberry Sheet Cake

  • 3/4 cup buttermilk room temperature
  • 2/3 cup sour cream room temperature
  • 3 large eggs room temperature
  • 1/3 cup vegetable oil
  • 1 Tablespoon vanilla
  • zest of one lemon (about 1 Tablespoon)
  • 1 15.25oz Lemon Cake Box Mix *Duncan Hines
  • 2 cups fresh blueberries

Lemon Curd (double batch) - there will be a little left over

  • 2 large eggs
  • 6 egg yolks
  • 1 1/2 cup granulated sugar
  • 1 cup lemon juice
  • 1/4 cup lemon zest
  • 1/2 cup unsalted butter

Easy Blueberry Buttercream

  • 1 1/2 cup unsalted butter left out at room temperature for 10 minutes
  • 1 Tablespoon vanilla
  • 1 pinch salt
  • 1 Tablespoon Blueberry Emulsion
  • 1/2 cup heavy cream + more to thin if needed
  • 7-8 cups powdered sugar

Garnishes

  • Fresh Lemon Slices

Instructions

For the Lemon Blueberry Cake

  • Preheat the oven to 325 degrees (convection bake) and prep a 9x13 pan with a little baking spray or shortening + flour to prevent sticking. Set aside.
  • In a large bowl, hand whisk together all the ingredients except for the cake mix. Sift in the cake mix and fold in gently until combined. Don't overmix. Toss in the blueberries and gently fold them in. Spread the batter into the prepared baking dish. Bake for 25-27 minutes until the center is baked through. Set aside to cool completely.

For the Lemon Curd *Make in advance up to a few hours to overnight to set

  • In a medium saucepan over medium heat, whisk together the eggs, sugar, lemon juice, and zest until the mixture thickens, about 3-5 minutes (keep whisking! we don't want scrambled eggs!). Remove the curd from the heat and add in the butter, 1 tablespoon at a time. Strain the curd to remove the zest. Transfer to a container to store in the fridge for a couple hours to cool and thicken - I usually do mine overnight.

For the Easy Blueberry Buttercream

  • In a stand mixer fitted with a paddle attachment, whip up the butter until it's light and fluffy, then add in the vanilla, salt, and blueberry emulsion. Add in the heavy cream, then toss in the powdered sugar with the mixer on low speed. If it's too thick, add in a tiny bit more heavy cream. Turn the mixer up to high speed and whip up for 2 minutes until the buttercream is light and fluffy. Use a wooden spoon to beat out the air bubbles before adding to the cake.

Assembly

  • On the cooled cake, spread the cooled/thickened lemon curd in a flat layer. Place into the freezer for 5 minutes to firm up a bit. Pipe on the blueberry buttercream (I didn't use a tip, just a piping bag with the end snipped off), and then spread flat with an offset icing spatula. On one half of the cake, add on the lemon slices and blueberries for garnish/decoration. Enjoy!