INGREDIENTS
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3/4 cup (1 1/2 sticks) butter, softened
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3/4 cup granulated sugar
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3/4 cup firmly packed light brown sugar
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1 large egg
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1 teaspoon vanilla extract
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2 1/4 cups all-purpose flour
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1/3 cup unsweetened cocoa powder
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1 teaspoon baking soda
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1/2 teaspoon salt
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1 cup M&Ms chocolate candies (plus more for decorating the tops)
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1 cup coarsely chopped walnuts (optional)