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Preheat oven to 350 degrees and line a 9x9 or 11x7 baking dish with non-stick foil or grease well with baking spray. (The 9x9 pan will yield thicker bars while the 11x7 will be slightly thinner. The ones pictured above were made in a 9x9 pan.)
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In a bowl, cream butter, brown sugar, and granulated sugar together until well combined.
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Add in eggs, one at a time, beating well after each addition.
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Add in vanilla extract and mix.
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Next, add in baking soda, salt, all purpose flour, and cocoa powder. Mix until well combined.
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Lastly, add in chocolate chips and chopped oreo cookies and mix until combined.
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Spread half of the cookie dough evenly in the bottom of the pan.
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Spread marshmallow fluff on top of the dough as evenly as possible.
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Press or spread the remaining cookie dough on top of the marshmallow fluff in an even layer (it’s fine if some of the marshmallow fluff is showing).
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Bake for about 25 minutes, or until the cookie bars have risen and the center is not jiggly when the pan is moved gently.
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Allow to cool at least 30 minutes before slicing. I prefer waiting at least an hour before cutting into them, or even refrigerating them and then cutting into them. By the way, these taste fantastic cold!
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Store remaining bars in an airtight container. They are fine at room temperature, if it’s not too warm, for 3-4 days. Or, you can refrigerate them as well.
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Enjoy!