Preheat your oven to 350°F (175°C) and prepare three 8-inch round cake pans by greasing them and lining the bottoms with parchment paper.

In a stand mixer, beat together 1 cup of unsalted butter, 1 cup of mascarpone cheese, and 2 cups of granulated sugar on medium speed until light and fluffy, about 4–5 minutes. Add in the zest of 2 lemons and 1 teaspoon of vanilla extract, mixing until well combined. Gradually incorporate 6 egg whites.

In a separate bowl, whisk together 3 cups of cake flour, 1 tablespoon of baking powder, and 1/2 teaspoon of salt.

Slowly add 1 cup of whole milk to the batter and then gradually incorporate the dry ingredients, mixing on low speed until smooth.

Evenly distribute the batter across the three prepared cake pans and bake for 25–28 minutes until golden and a toothpick comes out clean.

Allow the cakes to cool in the pans for 10 minutes before transferring to wire racks to cool completely.

In a clean bowl, beat together 1 cup of unsalted butter and 1 cup of mascarpone cheese until creamy. Gradually add 4 cups of powdered sugar, lemon zest, blueberry powder, and 2 tablespoons of milk until you reach the desired consistency.

Level the cooled cake layers. Spread frosting on the first layer, add the second layer with frosting, and then place the third layer on top, applying a crumb coat to seal in crumbs.

Chill the cake for about 30 minutes to set, then use remaining frosting to cover the cake evenly and decorate with fresh lemon slices and blueberries.