Preheat your oven to 180°C (350F). Place paper liners in a cupcake tin.
In a medium bowl, sift the dry ingredients together cocoa powder, flour, baking soda, baking powder, and salt.
In a bowl beat the eggs, sugar, brown sugar, vegetable oil, and vanilla extract until combined well.
Start adding dry mix to the wet mix alternating it with the buttermilk. Mix till everything is combined. Don't over-mix. The batter consistency will be somewhat thin.
Pour batter into the prepared cupcake tin. Fill the molds (paper liners) till halfway filled. Bake for 18-20 minutes, or until a toothpick inserted in the center comes out clean.
Let it cool completely before frosting.
Frosting and hot chocolate mix- (the hot chocolate mix is added in the frosting)
In a bowl take the heavy cream and heat it in the microwave for 20-30 seconds or till the heavy cream melts. Don't overheat.
Add the cocoa powder, salt, sugar, and milk powder to it and mix it properly.
In another bowl take the butter and beat until smooth for about 2-3 minutes. Slowly start adding in the sugar in batches and beat till you get that light creamy buttercream frosting texture.
Start adding the Hot cocoa mixture to it and beat. If you feel your frosting is a little runny, add more sugar.
Pipe the frosting on your cupcakes after they are cooled. Top your hot chocolate cupcakes with marshmallows and enjoy!
Notes
** You're welcome to cheat and use a rounded 1/2 cup of your favorite hot chocolate mix!