Preheat oven to 350°F (175°C). Line a muffin tin with cupcake liners.

In a large bowl, whisk together flour, sugar, cocoa powder, baking soda, baking powder, and salt.

Add buttermilk, oil, egg, and vanilla to the dry ingredients. Beat with an electric mixer on medium speed for 2 minutes.

Gradually add hot water, mixing until just combined. The batter will be thin.

Fill cupcake liners about 2/3 full.

Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean. Let cool in the muffin tin for 5 minutes before transferring to a wire rack to cool completely.

In a large bowl, beat butter with an electric mixer until light and fluffy.

Gradually add powdered sugar, beating on low speed until combined. Add milk and vanilla extract, and beat on medium speed until smooth and creamy.

Divide frosting in half. Tint one half red with food coloring.

Frost cupcakes with white frosting. Pipe a swirl of red frosting on top of each cupcake in a cone shape.

Place a mini marshmallow on top of the red frosting to create the "pom-pom" of the Santa hat.