INGREDIENTS
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½ cup butter, softened
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½ cup sugar
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½ cup brown sugar
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1 egg, at room temperature
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1 tsp vanilla extract
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1 cup all-purpose flour
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½ cup cocoa powder
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1 tsp baking soda
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¼ tsp salt
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2 Tbls milk
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1 cup mini M&M's, plus ¼ cup for the top