Preheat your oven to 350°F (175°C). Grease three 8-inch round baking pans and line with parchment paper.

In a stand mixer, cream together granulated sugar, unsalted butter, and mascarpone cheese until light and fluffy (3-5 minutes). Mix in vanilla extract and lemon zest.

Gradually add egg whites, mixing just until combined.

In a separate bowl, whisk together cake flour, baking powder, and salt. Gradually add to wet ingredients, mixing gently until just combined.

Fold in whole milk until smooth and thick.

Divide batter among prepared pans and bake for 25-28 minutes, or until a toothpick comes out clean.

Let the cakes cool in the pans for 10 minutes, then remove and cool completely on a wire rack.

Beat unsalted butter and remaining mascarpone until smooth. Gradually add powdered sugar, then mix in milk, lemon zest, and freeze-dried blueberries.

Assemble the cake by leveling layers if needed and adding frosting between layers, then cover the entire cake with frosting.

Decorate with fresh lemon slices, blueberries, and lemon zest. Serve chilled.