In a medium saucepan, combine frozen berries, sugar, lemon juice, zest, and optional mint leaves. Heat over medium-low, stirring frequently, until the mixture simmers. Allow it to cook for about 20 minutes until it thickens and becomes syrupy. Once thickened, remove from heat and let it cool completely before refrigerating it to set.

Preheat your oven to 350°F (175°C) and grease two 8-inch round cake pans. In a large mixing bowl, cream together room temperature unsalted butter, white sugar, and neutral oil until fluffy and light. Incorporate eggs one at a time, mixing well after each addition, followed by vanilla extract.

Gradually combine baking powder, flour, and salt with the wet mixture, alternating with buttermilk until you achieve a smooth batter.

Divide the cake batter evenly into the prepared pans and smooth the tops with a spatula. Bake in the preheated oven for approximately 30-35 minutes or until a toothpick inserted into the center comes out clean. Let the cakes cool in the pans for 10-15 minutes before transferring them to wire racks to cool completely.

In a chilled mixing bowl, whip the heavy cream, granulated sugar, and a pinch of salt using an electric mixer until soft peaks form. Gently fold in the cold mascarpone cheese until fully combined and stiff peaks are achieved.

Start by placing one layer of cooled vanilla cake on a serving plate. Spread a generous layer of whipped mascarpone frosting on top, pipe a decorative border, then add a layer of the mixed berry compote. Carefully place the second cake layer on top and frost the entire cake, finishing with fresh berries.