For the Blueberry Filling:

• 1 cup frozen blueberries (can use fresh, but frozen yields more juice)
• 1/4 cup water
• 1 Tablespoon fresh lemon juice (from about half a lemon)
• 1 Tablespoon lemon zest (finely grated, for an extra citrus kick)
• 1 Tablespoon corn starch
• 1 Tablespoon cold water (for corn starch slurry)

For the Lemonade Cake:

• 1 box of white cake mix (your favorite brand)
• 3/4 cup frozen concentrate lemonade (thawed slightly)
• 1/2 cup water
• 4 large egg whites (at room temperature for best results)
• 1/3 cup vegetable oil

For the Lemonade Buttercream Frosting:

• 6 cups powdered sugar (sifted for a smoother frosting)
• 2 sticks (1 cup) unsalted butter (softened to room temperature)
• 3 Tablespoons frozen lemonade concentrate (thawed)
• Pinch of salt
• Yellow food coloring (optional, for a brighter lemon color)
• Fresh blueberries (for garnish, optional)

Blueberry Filling:

• In a small saucepan, combine the frozen blueberries, 1/4 cup water, 1 Tablespoon lemon juice, and 1 Tablespoon lemon zest. Bring the mixture to a gentle boil over medium heat, then reduce the heat to low and simmer for 10-15 minutes, stirring occasionally. The blueberries will break down and release their juices.
• While the blueberries simmer, prepare a corn starch slurry by dissolving 1 Tablespoon of corn starch in 1 Tablespoon of cold water in a separate small bowl. Once the blueberry mixture has simmered, gradually add the corn starch slurry to the saucepan, stirring constantly. Continue to stir until the filling thickens, which should take about 5 minutes. It should coat the back of a spoon.
• Remove the blueberry filling from the heat and transfer it to a heatproof bowl. Cover it with plastic wrap, pressing the wrap directly onto the surface of the filling to prevent a skin from forming. Place the bowl in the refrigerator for at least 24 hours (or overnight) to allow it to cool completely and thicken to the ideal spreading consistency. This step is crucial for a stable cake filling.

Lemonade Cake:

• Preheat your oven to 350°F (175°C). In a large mixing bowl, combine the white cake mix, 3/4 cup frozen lemonade concentrate, 1/2 cup water, 4 egg whites, and 1/3 cup vegetable oil. Begin by stirring the ingredients together on low speed with an electric mixer until just combined. Then, increase the speed to medium and beat for 2 minutes, making sure to scrape down the sides of the bowl halfway through to ensure all ingredients are well incorporated. Avoid overmixing, as this can lead to a tough cake.
• Prepare your baking pans by thoroughly greasing and flouring them. For best results with a layered cake, use two 8-inch or 9-inch round cake pans. A light coating of baking spray with flour also works wonderfully for easy release.
• Divide the cake batter evenly between the prepared pans. Bake for 20-25 minutes, or until a wooden toothpick inserted into the center of each cake comes out clean. Baking times can vary, so keep an eye on your cakes.
• Allow the cakes to cool in their pans on a wire rack for about 10-15 minutes before inverting them onto the wire rack to cool completely. It is absolutely essential that the cakes are entirely cool before frosting, otherwise, your buttercream will melt and slide off.

Lemonade Buttercream Frosting & Assembly:

• In a large mixing bowl, using an electric mixer, beat the softened unsalted butter on medium speed until creamy and light, about 2-3 minutes. Gradually add the sifted powdered sugar, one cup at a time, beating well after each addition. Once all the sugar is incorporated, add the thawed lemonade concentrate and a pinch of salt. Beat on high speed for another 3-5 minutes until the frosting is light, fluffy, and thick. If desired, add a few drops of yellow food coloring for a brighter lemon hue. If the frosting is too thick, add a tiny bit more lemonade concentrate or milk (1 teaspoon at a time); if too thin, add more powdered sugar.
• To assemble, place one cooled cake layer on your serving plate or cake stand. Spread about half of the prepared blueberry filling evenly over the top of this layer. Some filling may drip down the sides; don’t worry, you can gently clean this up with a paper towel later. Place the second cake layer carefully on top of the filling.
• Apply a thin “crumb layer” of frosting over the entire cake (top and sides). This layer is important for sealing in any loose crumbs and creating a smooth base for your final frosting coat. Once the crumb coat is applied, place the cake in the refrigerator for about 20-30 minutes to allow this layer to firm up. This step helps ensure a clean, beautiful finish.
• After the crumb coat has chilled, frost the cake with the remaining Lemonade Buttercream. You can apply it smoothly with an offset spatula for a classic look, or get creative with decorative piping. For our celebration cake, I piped a lovely edge around the bottom and created a decorative ring of piped frosting in the center of the top. Into this central ring, I artfully placed fresh blueberries, adding a beautiful pop of color and freshness.
• For an extra touch of elegance and flavor, arrange additional fresh blueberries around the base of the cake on the serving plate. Slice, serve, and enjoy this delightful, refreshing summer cake!