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Preheat oven to 350°F. Line a standard muffin pan with paper cupcake liners.
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Make the cupcakes: In a large bowl, whisk together the flour, cocoa powder, baking powder, baking soda, sugar, and salt together.
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In the bowl of stand mixer or using a hand mixer, beat together the egg, milk, oil, peppermint, and vanilla. Slowly add the flour mixture and beat on medium speed with an electric mixer for 30 seconds. Scrape down the sides of the bowl and continue mixing on medium speed for 1 to 2 minutes. Stir boiling water into batter until well combined. Fold in mint chocolate chips.
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Divide the batter equally into prepared muffin tin filling to about three-quarters full. Bake for 17-19 minutes or tops are firm and a cake tester inserted in the center comes out clean Remove pan from oven and allow cupcakes to cool in pan for 10 minutes. Remove cupcakes from pan and let cool completely before frosting.
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Meanwhile, make the frosting: Beat softened butter with a hand or stand mixer on medium speed. Beat for about 3 minutes until smooth and creamy.
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Add powdered sugar and mix until combined.
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Add vanilla, peppermint extract and heavy cream and mix until smooth.
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If necessary, add more heavy cream or powdered sugar until desired consistency is reached.
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To assemble, cut each cupcake in half horizontally. Spread an even layer of frosting on the bottom of the cupcake. Replace top of cupcake over the frosting, pushing down slightly to flatten. Pipe more frosting on top of the cupcake and sprinkle with crushed Oreo cookies if desired. Serve immediately