Preheat oven to 350F. Grease a mini-muffin tin (24), set aside.
For the Brownies:
In a medium-sized glass bowl set on top of a pot of simmering water, melt the chocolate and butter. Stir together until smooth.Remove from heat and whisk in the sugar. Add the eggs one at a time and whisk in until smooth. Stir in the vanilla. Pour the flour, salt, and baking powder over the wet ingredients. Stir until smooth and well-combined (be careful not to over mix).
For the Cheesecake Batter:
Beat together the softened cream cheese with the egg yolk and sugar until super smooth and clump free.
To Assemble:
Fill each muffin well 1/2 way full of brownie batter. Pipe a layer of cheesecake batter on top of the brownie, and then add another large dollop of brownie batter. Use a fork or toothpick to swirl the batters together.
Bake for 15 minutes, or until a toothpick inserted comes out clean. Enjoy!