For the Brownie Base: Adjust an oven rack to the lower-middle position and heat the oven to 325 degrees. Spray an 8-inch square baking pan with nonstick spray. Line with parchment paper and spray the parchment paper as well.
Whisk the flour, salt, and baking powder together in a small bowl. Set aside.
In a medium heatproof bowl set over a pan of almost simmering water, melt the chocolates and butter, stirring occasionally until the mixture is smooth. Remove from the heat and whisk in the sugar and vanilla.
Let cool slightly. Whisk in the eggs one at a time, fully incorporating each before adding the next. Continue whisking until completely smooth.
Add the dry ingredients and whisk until just incorporated.
For the Cream Cheese Filling: In a small bowl, beat the cream cheese with the sugar, vanilla, and egg yolk until evenly blended.
Pour half the brownie mixture into the pan. Drop half the cream cheese mixture, by spoonfuls, over the batter. Repeat, layering the remaining brownie mixture and cream cheese mixture. Use the blade of a table knife to gently swirl the brownie batter and cream cheese filling, creating a marbled effect.
Bake until the edges of the brownies have puffed slightly, the center feels not quite firm when touched lightly, and a toothpick inserted into the center comes out with several moist, fudgy crumbs adhered to it. About 50 or 60 minutes.
Cool the brownies in the pan on a wire rack for 5 minutes. Remove from the pan with the parchment and cool to room temp on the wire rack. Refridgerate until chilled - like 3 hours.