Preheat the oven to 350°F (175°C) and place the oven rack in the middle.

Line a 9x13 inch pan with parchment paper, leaving an overhang around the edges.

In a medium bowl, whisk together the flour, baking powder, and salt.

In a separate large bowl, beat together the softened butter, brown sugar, and white sugar until soft and fluffy.

Beat in the peanut butter until well incorporated, then mix in the egg and egg yolk until smooth.

With the mixer on low speed, mix the flour mixture into the peanut butter mixture about 1/2 at a time until glossy and smooth.

Remove about 1 cup of batter from the bowl, forming a sticky dough.

Press the rest of the peanut butter dough into the bottom of the lined pan, forming an even layer.

Bake the peanut butter layer in the preheated oven for approximately 25 minutes, or until the top looks fully set.

Carefully remove the pan from the oven, keeping it turned on, and spread the marshmallow fluff evenly on top.

Pinch off pieces of the reserved peanut butter cookie dough, flatten them slightly, and place them on top of the marshmallow layer—80 to 90% coverage is ideal.

Place the pan back in the oven and bake for another 10 minutes, or until the peanut butter layer on top is set.

Once baked, remove the pan from the oven and let the bars cool at room temperature for at least an hour before slicing.

Lift the bars out of the pan using the parchment overhang and place them onto a cutting board.

Lightly grease a thin, sharp knife with non-stick cooking spray and slice the bars evenly.