Preheat the oven to 350℉. Line a 9x9-inch baking pan with parchment paper.

In a mixing bowl using an electric mixer, cream together the softened butter, brown sugar, and white sugar for 30 seconds. Add the egg and vanilla and continue creaming at high speed until fluffy.

In a separate bowl, whisk together the flour, baking soda, baking powder, and salt. Add to the wet ingredients and continue mixing at low speed just until incorporated. Spread about ⅔ of the mixture into the bottom of the prepared baking pan, smoothing it into an even layer.

Add the marshmallow fluff on top of the batter and spread all around with a greased spatula as best as you can. Lastly, top with dollops of the remaining batter all over on top of the marshmallow fluff. Spread the batter out as best as you can.

Bake the bars in the preheated oven for 25-30 minutes or until lightly golden around the edges and puff up in the center. Let cool slightly before lifting the bars out of the pan using the parchment paper as a handle. Slice and serve. Enjoy!