Preheat oven to 350F. Grease a mini-muffin tin and 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.
Sprinkle the sea salt, flour and baking powder over the wet ingredients. Stir until smooth and well-combined but don't 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 a pastry bag with the cheesecake batter and cut a small opening at the tip. 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 skewer or toothpick to swirl the batters together. (The batter can reach just about the top of the well, but not above it.)
Bake for 15-20 minutes, or until a toothpick inserted comes out clean.