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1
Preheat oven to 350 F. Spray a square (8x8 or similar) baking dish with non-stick spray and set aside for now.
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2
Put the butter into a medium-sized microwave-safe bowl. Heat it in the microwave until melted. Start with 30 seconds and add time if needed. Let it cool so it's not super hot, about five minutes.
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3
While it's cooling, whisk together your dry ingredients in a small bowl (flour, baking soda and salt) and set aside.
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4
Whisk the egg, peanut butter, sugars and vanilla into the butter until thoroughly combined. Now add your flour mixture into the dough until just combined, but being careful not to over-mix.
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5
Fold in your Marshmallow Fluff in tablespoon-sized dollops. Again, don't over-mix or the stuff will just disappear. You want to see some streaks and pockets of marshmallow.
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6
Spread the batter into your prepared pan, smoothing the top. Bake for 23-28 minutes until the center is just set. A toothpick will come out mostly clean when it's done. You don't want to over-bake because they're best gooey, but you don't want too gooey either, so it's a delicate balance. When done remove dish from oven and set on a rack.
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7
Let it cool completely, then cut it into bars and go to town! These will probably taste even better on day two once everything has melded together even more.
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8
Adapted from a recipe by Bakerita.
