• Preheat the oven to 350℉ (180℃). Line an 8x8 inch (20x20 cm) square baking pan with parchment paper, leaving an overhang around the sides.

Make the Peanut Butter Graham Cookie Dough

• In a medium bowl, whisk together the melted butter, peanut butter and sugar. When finished, there should be no lumps of brown sugar and the melted butter shouldn't sit on top of the sugar.
• Whisk the egg, additional egg yolk and vanilla extract into the butter mixture until you no longer see pieces of egg.
• Stir in the flour and salt. Keep stirring until there are no lumps of flour.
• Stir in the graham crumbs.

Assemble & Bake

• Scoop out about 1 cup of cookie dough. Save this dough - it's used for the top layer later.
• Press the remaining dough into the bottom of the prepared pan, creating an even layer.
• Place a layer of chocolate on top of the cookie dough and press down slightly. You will need to break your chocolate bars in order to get the base completely covered, depending on their size.
• Microwave the peanut butter in a a heatproof bowl for about 30-45 seconds on medium speed. Stir the peanut butter until smooth, then spread over top of the chocolate layer. Microwaving first makes it easier to spread on top of the chocolate.
• Measure out the marshmallow fluff, add to a heatproof bowl and microwave for 30-45 seconds on medium power. Spoon the marshmallow fluff over the peanut butter layer and spread into an even layer. Again, microwaving the fluff first makes spreading it much easier.
• With the reserved 1 cup of cookie dough, form into flat discs and place on top of the marshmallow layer. You don't need to have all of the marshmallow covered, but try not to have too large of exposed areas.
• Bake in the middle of the preheated oven for about 30-25 minutes, or until the peanut butter cookie dough on top looks set and is golden brown. The marshmallow will have started to fluff up where it is exposed.
• Cool the bars in the pan. For clean cuts, the bars need to be completely cooled. When ready to slice, lift the bars out of the pan using the overhang of the parchment paper and place on a cutting board. Cut with a sharp knife. If the bars are still warm, they will fall apart. They will be deliciously gooey, but you'll need to eat them with a spoon.