Make the brownies:

• Pre heat oven to 350°. Line with non-stick foil (or lightly grease), a 9” x 13” brownie or cake pan.
• Whisk together the flour, peanut butter powder, cocoa powder and salt in a small bowl. Set aside.
  100 g flour, 12 g cocoa powder, 6 g salt, 30 g peanut butter powder
• Place the dark chocolate, unsalted butter, and espresso powder in a large microwave safe bowl. If your microwave has a chocolate melting feature, use that, otherwise, heat for 1 minute in the microwave, stir and then continue to heat in 30 second bursts, stirring after each, just until the chocolate is fully melted. (This can also be done over a double boiler.)
  340 g chocolate, 224 g unsalted butter, 1/2 teaspoon espresso powder
• While the chocolate and butter are melting, whisk the eggs, sugar and brown sugar until thick and ribbony (this is faster with an electric mixer, but can be done by hand). Whisk in the vanilla until combined.
  200 g sugar, 110 g brown sugar, 5 eggs, 15 ml vanilla
• Whisk the chocolate mixture into the egg mixture until a homogenous, glossy mixture is achieved. At this point the sugar should be fully dissolved with no grittiness in the mixture.
• Stir in the flour mixture just until it disappears. Fold in the peanut butter chips and chocolate chips.
  170 g peanut butter chips, 85 g milk chocolate chips
• Pour half of the batter into the prepared pan, smooth out the top. Arrange the peanut butter cups evenly over the surface of the batter. Top with the remaining batter, carefully spread to the edges of the pan ensuring all of the peanut butter cups are covered
  209 g Reese's Peanut Butter Cup Thins
• Pipe, spoon or dollop half of the peanut butter over the surface of the batter. Use a butter knife to gently swirl the peanut butter into the surface of the batter (the knife can only extend very slightly into the batter, you want to avoid disrupting the peanut butter cup layer - the goal is to make sure there are swirls of peanut butter in every bite of brownie)
  128 g peanut butter
• Place the pan in the pre-heated oven and bake for about 35 minutes. The top will appear set and glossy around the ribbons of peanut butter, the edges slightly puffed and possibly a crack or two in the surface. Place on a wire rack to cool completely.
• Spoon the marshmallow crème over the top of the cooled brownies, spread into an even layer all the way to the edges of the brownies. Pipe, spoon or dollop the remaining peanut butter over the surface of the marshmallow layer. Use a butter knife to gently swirl the peanut butter through the marshmallow layer (avoid hitting/digging into the brownie layer underneath)
  285 g homemade marshmallow crème, 128 g peanut butter
• For the cleanest cuts, chill the bars for at least an hour before cutting into 24 large bars, 32 medium bars or 40 small bars. Store in an air-tight container at room temperature.