Preheat oven to 300°F and prepare a cupcake pan with liners.

Mix flour, sugar, cocoa powder, baking soda, and salt in a large bowl.

Whisk egg, milk, oil, and vanilla extract in another bowl.

Combine wet and dry ingredients until smooth.

Add hot water and mix until the batter is thin.

Fill cupcake liners halfway and bake for 18-23 minutes.

Cool cupcakes on a rack after baking.

Make frosting by beating butter until smooth.

Mix melted chocolate into the frosting.

Add cocoa powder and mix well.

Gradually add powdered sugar and heavy cream to reach desired consistency.

Frost cooled cupcakes with chocolate frosting.