1 (15.25-oz.) box vanilla cake, plus ingredients called for on box
Juice and zest of 1 1/2 lemons, divided, plus more zest for garnish
1 3/4 c. fresh blueberries, divided
3 tbsp. all-purpose flour
1 c. (2 sticks) butter, softened
3 c. powdered sugar
1/4 c. heavy cream
1 tsp. pure vanilla extract
Pinch kosher salt
Thin lemon slices, for topping
Step 1
Preheat oven to 350º. Grease three 9" cake pans with cooking spray and line with parchment. Prepare cake mix according to package directions, then stir in juice and zest of 1 lemon.
Step 2
In a small bowl, toss 1 cup blueberries and flour until completely coated. Gently fold blueberries into the batter.
Step 3
Divide cake batter evenly among prepared cake pans and bake until a toothpick comes out clean, 18 to 20 minutes.
Step 4
Let cool in pans for 10 minutes, then invert onto a wire rack and let cool completely.
Step 5
Make frosting: In a large bowl using a hand mixer or the bowl of a stand mixer using the whisk attachment, beat butter and 2 1/2 cups powdered sugar. Add remaining lemon juice and zest and heavy cream and beat until combined, then beat in vanilla and salt. (Add remaining 1/2 cup powdered sugar as desired for texture and flavor.)
Step 6
Place a dab of frosting on cake plate to keep cake from moving and place a parchment strip on each side of the cake plate. Place down first cake layer and top with frosting, then repeat with the second and third cake layers and remaining frosting. Smooth frosting over the top and sides of your cake.
Step 7
Garnish with remaining blueberries, lemon slices, and zest before serving.