01 -
Preheat the oven to 175°C and position the oven rack in the middle.
02 -
Line a 23x33 cm pan with parchment paper, leaving an overhang, or lightly grease aluminum foil.
03 -
In a medium bowl, whisk together flour, baking powder and salt until evenly mixed.
04 -
In a large bowl, beat the softened butter with brown and white sugars until soft and creamy.
05 -
Mix in the peanut butter, followed by the egg and egg yolk, until the mixture is smooth.
06 -
With the mixer on low speed, gradually add the flour mixture to the peanut butter mixture, mixing halfway at a time.
07 -
Spoon about 1 cup (approximately 240 grams) of batter into a separate bowl.
08 -
Press the remaining dough evenly into the bottom of the prepared pan to form a uniform layer.
09 -
Bake the peanut butter layer in the preheated oven for 25 minutes, or until the surface appears fully set.
10 -
Remove the pan from the oven and keep the oven on. Carefully spread the entire jar of marshmallow fluff evenly over the baked layer.