1-First, prepare the berry filling: Simmer 2 cups of mixed berries with 1/2 cup granulated sugar and 1 tablespoon water in a pot over medium heat until the berries break down.

2-Thicken the mixture by stirring in a slurry of 1 tablespoon cornstarch mixed with a little water, then add 1 tablespoon fresh lemon juice and 2 teaspoons lemon zest. Let it cool completely before using.

3-Preheat your oven to 350°F (175°C) and prepare your cake pans by greasing them. In a large bowl, whisk together 3 cups cake flour, 3 cups granulated sugar, 2 1/2 teaspoons baking powder, and 1 teaspoon fine salt.

4-Gradually add 1 cup unsalted butter to the dry mix using a mixer until it resembles moist sand. Lightly mix in 1 cup egg whites, then add 1 1/2 cups sour cream, 2 tablespoons oil, and 1 teaspoon vanilla extract until smooth.

5-Bake the cake layers for 33-36 minutes or until a toothpick comes out with moist crumbs. Cool the cakes, then freeze the pans for 45 minutes and level the tops with a serrated knife.

6-For the frosting, chill your mixing bowl and whisk. Beat 1 cup cream cheese until smooth, then gradually add 1 cup powdered sugar and 1 teaspoon vanilla extract. Slowly incorporate 2 1/2 cups heavy whipping cream in parts on medium-high speed until stiff peaks form.

7-Assemble the cake: Place a layer on a board, pipe a frosting ring around the edges, fill with the berry mixture and optional fresh berries, then cover with frosting. Repeat for remaining layers, apply a thin coat for a semi-naked look, chill, and decorate with fresh berries, mint, and edible flowers.