Preheat oven to 350°F (175°C) and line a muffin tin with paper liners. In a medium bowl, whisk together the flour, cocoa powder, baking soda, baking powder, and salt. In a separate bowl, beat the butter and sugar until light and fluffy. Add eggs one at a time, then mix in vanilla and sour cream. Stir in the melted chocolate and hot water until combined.

Gradually add the dry ingredients to the wet mixture. Stir until smooth but do not overmix. Divide the batter evenly among the cupcake liners, filling about ¾ full. Bake for 18–20 minutes or until a toothpick inserted comes out clean. Let cool completely.

In a cold mixing bowl, beat the heavy cream, powdered sugar, and vanilla extract until stiff peaks form. Spoon or pipe the whipped topping onto each cooled cupcake.

Garnish with mini marshmallows, a drizzle of chocolate sauce, or a sprinkle of shaved chocolate. Serve immediately or store chilled.