Preheat oven to 350°F. Line an 8x8-inch pan with parchment paper, leaving a 2-inch overhang along each side.

Combine chocolate and butter in a heatproof bowl set over a saucepan with ½-inch of simmering water and stir until completely melted, smooth and glossy. Or, melt chocolate with butter in the microwave in short 20 second bursts with frequent stirring.

Combine eggs with both sugars and vanilla in the bowl of a stand mixer fitted with the whisk attachment and beat for 4-5 minutes until pale, thick and fluffy. When you lift up the beaters, the egg foam will fold in ribbons back onto itself as it falls back into the bowl. Beat in salt then mix in the warm melted chocolate mixture until evenly blended. Combine flour and cocoa in a small bowl to press out any lumps then sprinkle it over the batter and fold it in until well incorporated and smooth. Spread ¾ of the batter into prepared pan and set aside.

For the cheesecake mixture, beat soft cream cheese with sugar until smooth and glossy. Mix in egg yolk and then stir in peanut butter. Dollop this mixture randomly over the chocolate batter. Add spoonfuls of the reserved brownie batter on top and swirl it all together with a skewer. Bake for about 25 minutes until a skewer inserted into the center comes out with a few moist crumbs attached. Transfer pan to a wire rack and let cool completely. Refrigerate for 1 hour before slicing into squares.