Preheat the oven to 350°F. Line an 8 by 8-inch light-colored metal pan with parchment or foil, leaving an overhang on all sides, then spray with nonstick baking spray.

In a large microwave-safe bowl, melt the butter. Add the brown sugar and peanut butter, stirring until smooth. Set aside to cool.

In a medium mixing bowl, combine the flour, graham cracker crumbs, baking powder, baking soda, and salt. Set aside.

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. Dough will be fairly thick.

Press two-thirds of the dough into the prepared pan, spreading evenly to the edges of the pan. 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, don’t worry about making it perfect. Sprinkle the chocolate chips evenly over the marshmallow.

Using your hands, pick up portions of the remaining dough and flatten with your palms. Top the marshmallow and chocolate chips with pieces of the flattened dough to cover about two-thirds of the top.

Bake for about 26-28 minutes or until the center is slightly jiggly, the edges are lightly golden brown, and the marshmallow on top is puffed and lightly golden brown. Be careful not to overbake to avoid dry blondies. Allow to cool in the pan completely. Cut into squares and serve.

Store leftovers in an airtight container for up to 3 days.