Preheat the oven to 350 degrees F (175 degrees C). Grease two 6-inch cake pans and line with parchment paper.
Combine butter and sugar in a bowl and beat using an electric mixer until creamy, about 5 minutes. Add eggs, 1 at a time, beating well after each addition. Gradually sift in flour and continue mixing until batter is smooth. Add vanilla extract and whisk for an additional 2 minutes. Divide batter between the prepared cake pans.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 40 minutes. Cool on a wire rack for 5 minutes. Run a table knife around the edges to loosen. Invert carefully onto a serving plate or cooling rack. Let cool completely, about 30 minutes.
While cake is cooking beat cream, mascarpone cheese, 2 tablespoons sugar, and 1 teaspoon vanilla extract in a bowl using an electric mixer until smooth, creamy, and thick. Cover bowl and refrigerate filling until cake is ready for decorating.
Once cakes are cooled completely, carefully cut each in half horizontally to create 4 layers. Set aside a handful of raspberries and blackberries for garnish. Mix remaining raspberries, blackberries, and strawberries together in a bowl.
Place 1 cake layer on a serving platter and cover with 1/3 of the mascarpone cream. Sprinkle 1/3 of the berries on top. Add the next cake layer and top with 1/3 of the mascarpone cream and 1/3 of the berries; repeat with remaining 2 layers, mascarpone cream, and berries. Garnish with fresh mint leaves and reserved blackberries, raspberries, and strawberries. Dust cake top with confectioners' sugar.
