Prepare your oven and supplies. Preheat your oven to 350°F (175°C) and line a standard muffin tin with 12 cupcake liners.
Mix the dry ingredients. In a medium bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt until well combined and free of lumps.
Combine the wet ingredients. In a large bowl, beat the eggs, granulated sugar, and brown sugar with an electric mixer until light and fluffy, about 2 minutes. Add the oil and vanilla extract, beating until incorporated.
Alternate adding dry ingredients and buttermilk. Gradually add the dry ingredients to the wet mixture in three additions, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix just until combined after each addition – don’t overmix!
Divide and bake the batter. Fill each cupcake liner about 2/3 full with batter. Bake for 18-22 minutes or until a toothpick inserted in the center comes out with just a few moist crumbs.
Cool completely before decorating. Allow cupcakes to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.
Make the chocolate frosting. Beat the softened butter in a large bowl until creamy, about 2 minutes. Gradually add the powdered sugar, cocoa powder, 3 tablespoons of heavy cream, vanilla, and salt. Beat until smooth and fluffy, about 3 minutes, adding more cream if needed for a spreadable consistency.
Frost each cupcake. Using an offset spatula or piping bag, apply a generous dome of chocolate frosting to each cooled cupcake, creating a smooth surface for decorating.
Create the reindeer faces. Gently press two pretzel twists into the top of each cupcake for antlers. Position two candy eyes below the antlers, and place a red candy in the center below the eyes for the nose.
Allow decorations to set. Let the cupcakes sit for about 15 minutes so the decorations can adhere firmly to the frosting before serving or storing.