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Preheat oven to 350 degrees and line a 9x9 or 11x7 baking dish with non-stick foil or grease with baking spray (9x9 will yield thicker bars while 11x7 will be slightly thinner). Set aside.
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In a bowl, mix butter and peanut butter together until combined.
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Add in brown sugar and granulated sugar, and mix until combined.
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Add in eggs, one at a time, stirring well after each addition.
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Mix in vanilla extract.
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Add in baking soda, salt, and all-purpose flour, and mix until combined.
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Lastly, add in Reeses peanut butter cups (chopped into small pieces) and chocolate chips, mixing until combined (dough will be thick).
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Spread half of the dough evenly over the bottom of the pan. Set aside the rest of the dough.
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Spread the marshmallow fluff on top of the dough (the marshmallow fluff is sticky, but spread it out as best you can).
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Press/spread the remaining dough on top of the marshmallow fluff in an even layer(it’s okay if some of the marshmallow fluff is showing).
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Bake for about 25 minutes, or until the top is lightly golden brown.
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Allow the bars to cool for at least 30 minutes (I prefer to cool for at least an hour) before cutting into them.
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Enjoy!
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Store remaining bars in an airtight container (they’re fine at room temperature, if it’s not too hot, for up to 4-5 days).