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Preaheat oven to 350 degrees. Spray 9x9 baking pan with nonstick cooking spray.
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In a stand mixer, mix brownie mix, oil, and water until combined. Stir in Nutella. Once mixed, fold in Reese's cup pieces.
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Pour batter into prepared baking pan. Bake according to package directions.
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Remove from heat and let cook completely before slicing.