Preheat oven to 350F. Line an 8-by-8-inch pan with aluminum foil and spray with cooking spray; set aside.

In a medium-sized microwave-safe bowl, melt the butter, 45 seconds to 1 minute. Cool the butter for about 5 minutes, and then add the egg, peanut butter, sugars, and vanilla. Whisk together thoroughly. Add the flour and stir until just combined, taking care not to over-mix or the bars will be tough. Fold in tablespoon sized dollops of marshmallow cream, don't overmix or the fluff will disappear into the batter. You just want some streaks and some small dollops of fluff.

Spread batter into prepared pan, smoothing it lightly with a spatula or offset knife. Bake for about 15 to 20 minutes, or until center is just set. A toothpick should come out mostly clean. Don't overbake; these are meant to be gooey and firm up as they cool. Let cool completely, slice, and serve!