Preheat the oven to 355 F.
Combine the chocolate and butter in a medium heatproof bowl and set the bowl in a wide skillet of barely simmering water. Stir from time to time until the mixture is melted. Remove from the heat, add sugar, flour and baking powder and mix the ingredients to bring everything together.
Pour ¾ of the brownie batter into the lined baking pan.
In a separete medium bowl, whisk the mascarpone with the egg, vanilla extract and the remaining ¼ cup of sugar. Evenly spread the mascarpone mixture on top of the brownie batter and cover with the remaining batter. Carefully swirl the mixture a few times using a fork.
Bake for 30 minutes.
Let the brownies cool before cutting.