Instructions

• Preheat the oven to 350°F (175°C). Line two muffin tins with 16–18 paper liners.
• Prepare the red velvet cupcake batter according to the instructions on the cake mix box.
• Divide the batter evenly among the prepared liners, filling each about three-quarters full.
• Bake 18–22 minutes, or until a toothpick inserted in the center of a cupcake comes out clean. Remove from oven and cool the cupcakes completely before frosting.
• While the cupcakes cool, make the buttercream. Place the 3 cups room-temperature unsalted butter in a large mixing bowl.
• Using a mixer, beat the butter on medium speed until pale and smooth, about 30 seconds.
• With the mixer on low, add the 6 cups sifted powdered sugar one cup at a time, pausing to scrape down the sides of the bowl between additions.
• Add the 2 teaspoons vanilla extract and 1/4 teaspoon salt; beat on low speed for 15–20 seconds to incorporate.
• Increase the mixer speed to high and beat until the frosting is light and smooth, about 5 minutes. (The frosting may look very soft at first but will increase in volume and stiffen.)
• Set aside about 1 cup of the finished buttercream in a separate bowl. Add black food coloring to that reserved cup and mix until the color is evenly combined.
• Fit a piping bag with a small round tip and fill it with the black buttercream. Fit a second piping bag with a star or large round tip and fill it with the remaining (untinted) buttercream.
• Pipe swirls of the untinted buttercream onto each cooled cupcake.
• Using the black buttercream in the small round tip, pipe a thin "string" around the surface of each frosted cupcake to resemble a light strand.
• Press M&M's vertically into the side of the frosting along the black string to look like Christmas lights, spacing them as desired so they stick securely.