How to Make It
Preheat the oven to 325°. Butter and flour a 9-inch-square baking pan. In a small bowl, whisk together the flour, baking powder and salt. In a medium saucepan, melt the semisweet and unsweetened chocolate with the butter over low heat. Remove from the heat and whisk in 1 cup of the sugar and 1 tablespoon of the vanilla. Whisk in the whole eggs, one at a time. Whisk the dry ingredients into the chocolate mixture until the brownie batter is smooth.
In a bowl, whisk the mascarpone with the egg yolk and the remaining 1/4 cup of sugar and 1/2 teaspoon of vanilla. Pour half of the brownie batter into the baking pan. Dollop half of the mascarpone mixture on top and cover with the remaining batter. Dollop on the remaining mascarpone mixture and swirl the batter a few times with a knife to create a marbled effect.
Bake the brownies for about 55 minutes, until a toothpick inserted in the center comes out with a few crumbs attached. Let the brownies cool before cutting. Serve with the caramel corn and ice cream.