Preheat the oven to 350°F (175°C). Generously grease a 24-cup mini-muffin tin with a little of the extra butter and set the tin aside.

Prepare the brownie batter: place the 3 ounces unsweetened chocolate (roughly chopped) and 1 stick unsalted butter in a medium heatproof bowl set over a pot of simmering water (double boiler). Stir until the chocolate and butter are fully melted and smooth. Remove the bowl from the heat.

Whisk the 1 cup granulated sugar into the melted chocolate mixture until combined. Add the 2 large eggs one at a time, whisking well after each addition until the mixture is smooth. Stir in the 1 teaspoon vanilla extract.

Sprinkle the 1/3 cup all-purpose flour, 1/4 teaspoon salt, and 1/3 teaspoon baking powder over the wet ingredients. Gently stir until just combined and smooth—do not overmix.

Prepare the cheesecake batter: in a medium bowl, beat the 18-oz. package cream cheese (softened) with the 2 Tablespoons sugar and the 1 egg yolk until the mixture is very smooth and free of lumps.

Assemble the mini cheesecakes: spoon or pipe enough brownie batter into each greased muffin well to fill it about halfway.

Add cheesecake batter: pipe or spoon a layer of the cheesecake batter on top of the brownie layer in each well (enough to cover most of the brownie layer).

Top each well with another dollop of brownie batter so the brownie and cheesecake layers are visible.

Create swirls: use a toothpick or skewer to gently swirl the brownie and cheesecake batters in each well, making a marbled pattern without fully mixing the batters.

Bake the mini cheesecake swirl brownies in the preheated oven for about 15 minutes, or until the centers are set (a toothpick inserted in a center will come out with a few moist crumbs but not wet batter).

Let the tin cool on a wire rack for about 10 minutes. Then remove the mini brownies from the tin and allow them to cool completely on the rack before serving.