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Preheat oven to 350°F
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Line 2 cookie sheets with parchment paper.
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In a small skillet, melt 10 tbsp unsalted butter.
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Scrape with spatula continually, until the butter gets foamy on top and the butter smell gets deeper.
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Cook until the milk solids begin to brown, 2-4 minutes.
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Transfer to a bowl, add the remaining 4 tbsp of cold butter, and set aside to cool, 15 minutes.
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In a pie dish, combine 1/4 c packed brown sugar with white sugar, rubbing with fingers to break up the brown sugar. Set aside.
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In a large bowl, whisk together flour, baking soda and baking powder
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Add remaining 1 3/4 c brown sugar and eggs with cooled melted butter, along with vanilla and salt.
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Stir to combine until smooth.
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Add this to Flour mixture and stir until completely mixed
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Add in peanut butter chips and mini peanut butter cups
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Press mixture into square cake dish, spreading around until it's even.
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Sprinkle the brown sugar mixture over the top of the pan.
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Bake for 15 minutes, spinning the tray 180° halfway through.
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The pan may appear underdone, but you don't want it to overcook.
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Let sit until cooled.