01 -
Preheat the oven to 350°F (175°C). Line an 8 by 8-inch light-colored metal pan with parchment paper or foil, leaving an overhang on all sides for easy removal. Spray the lined pan with nonstick baking spray.
02 -
In a large microwave-safe bowl, melt the butter. Add the brown sugar and peanut butter, stirring until smooth. Set aside to cool slightly.
03 -
In a medium mixing bowl, combine the flour, graham cracker crumbs, baking powder, baking soda, and salt. Whisk to combine and set aside.
04 -
Whisk the egg and egg yolk into the cooled peanut butter mixture one at a time, whisking well after each addition. Add the vanilla extract and stir to combine. Add the flour mixture and stir until just combined. The dough will be fairly thick.
05 -
Press two-thirds of the dough into the prepared pan, spreading evenly to the edges.
06 -
Use a spatula to dollop the marshmallow creme around the base layer, then smooth as evenly as possible over the dough. It will be messy, but don't worry about making it perfect. Sprinkle the chocolate chips evenly over the marshmallow layer.
07 -
Using your hands, pick up portions of the remaining dough and flatten with your palms. Place these flattened pieces on top of the marshmallow and chocolate chip layers to cover about two-thirds of the surface.
08 -
Bake for 26-28 minutes or until the center is slightly jiggly, the edges are lightly golden brown, and any exposed marshmallow on top is puffed and lightly golden brown. Be careful not to overbake to avoid dry blondies.
09 -
Allow the blondies to cool completely in the pan. Once cooled, use the parchment overhang to lift them out, cut into squares, and serve.