Preheat oven to 350°F (180°C) and line two 12-cup muffin tins with cupcake liners.
In a large bowl, whisk together sugar, brown sugar, flour, cocoa powder, hot cocoa mix, baking soda, baking powder, and salt.
1 ¼ cups granulated sugar, ¾ cup brown sugar, 1 ¾ cups all-purpose flour, ¾ cup unsweetened cocoa powder, ¼ cup hot cocoa mix, 1 teaspoon baking soda, ½ teaspoon baking powder, 1 teaspoon salt
In another bowl, whisk buttermilk, oil, eggs, and vanilla extract until smooth.
1 cup buttermilk, ½ cup vegetable oil, 2 large eggs, 1 teaspoon vanilla extract
Pour the wet ingredients into the dry ingredients and mix until just combined. Gradually stir in hot water until the batter is smooth and slightly thin.
¼ cup hot water
Fill each cupcake liner about ¾ full. Bake for 18–20 minutes, or until a toothpick inserted in the center comes out clean. Allow cupcakes to cool completely before frosting.
In a large bowl, beat butter with a hand or stand mixer until light and fluffy. Gradually add confectioners’ sugar, mixing on low speed until combined.
1 cup unsalted butter, 4 cups confectioners’ sugar
Add heavy cream (or milk) and beat on medium speed until the frosting is light and creamy. Add a pinch of salt to balance the sweetness.
3 tablespoons heavy cream, Pinch of salt
Transfer frosting into a piping bag and swirl onto cooled cupcakes.
Dust with powdered sugar and top with mini marshmallows for a cozy hot chocolate touch.
Mini marshmallows