In a medium saucepan, combine mixed berries, lemon juice, sugar, and cornstarch over medium heat. Cook until thick and bubbly, approximately 5 minutes. Remove from heat and cool completely.

Preheat your oven to 350°F (175°C) and line two 9-inch round cake pans with parchment paper. Whisk together flour, cornstarch, baking powder, baking soda, and salt in a large mixing bowl.

In a separate bowl, beat unsalted butter, vegetable oil, and sugar until light and fluffy, about 3-4 minutes. Add eggs one at a time, mixing well, followed by vanilla and vinegar. Gradually mix in dry ingredients and buttermilk alternately.

Pour batter into prepared pans and bake for 25–30 minutes until golden and a toothpick inserted comes out clean. Cool in pans for 10 minutes, then transfer to a wire rack to cool completely.

Beat mascarpone cheese until smooth. In another bowl, whip heavy cream with powdered sugar until stiff peaks form. Gently fold whipped cream into mascarpone until fully incorporated.

Assemble the cake by spreading Chantilly cream over the first cake layer, followed by a layer of berry filling. Press the second layer of cake on top and frost with remaining Chantilly cream.

Garnish with fresh berries and chill in the refrigerator for at least 1 hour before serving.