Preheat the oven to 350ºF (177ºC). Line an 8″ x 8″ baking pan with foil, spray with non-stick cooking spray, and set aside.
In a medium size bowl, whisk together the cooled butter, eggs, peanut butter, and both sugars until smooth.
Add the flour to the wet ingredients and stir until just combined. Fold in the peanut butter chips until evenly dispersed.
Spoon in the marshmallow creme and use a spatula to “cut” the creme into the batter, being careful not to over-mix so the creme stays in dollops.
Pour the batter into the prepared pan and smooth it out with your hands. Add peanut butter chips and torn mini-marshmallows to the top, if desired, making sure to smooth out the batter again when you are finished.
Bake for 25-28 minutes or until a toothpick inserted in the middle of the pan comes out mostly clean. Allow to cool completely on a wire rack before cutting into 16 squares with a very sharp knife. Store in an airtight container at room temperature up to 4 days. Bars freeze well, up to 3 months. Thaw in the refrigerator overnight.