Classic Molten Chocolate Cake with Cassis Berries

Name: Classic Molten Chocolate Cake with Cassis Berries
Ingredients: 1/2 cup creme de cassis; 1 1/2 cups mixed berries; 1 stick (8 tablespoons) unsalted butter, plus more for the ramekins; 1 tablespoon cocoa powder, for dusting the ramekins; 4 ounces good-quality bittersweet chocolate, finely chopped; 1 teaspoon instant espresso powder; 1 teaspoon vanilla extract; 1 pinch salt; 1/2 cup all-purpose flour; 1/3 cup granulated sugar; 2 large eggs plus 2 yolks; 1 cup mascarpone; 1 teaspoon granulated sugar; Fresh mint sprigs, for garnish