Preheat your oven to 350°F (175°C) and line a muffin pan with cupcake liners.

In a large mixing bowl, whisk together the flour, sugar, cocoa powder, baking soda, and salt until thoroughly combined.

In a separate bowl, mix the wet ingredients: Blend the egg, milk (or buttermilk), vegetable oil, and vanilla extract until smooth.

Gradually incorporate the wet ingredients into the dry, whisking until just combined.

Pour in the hot water and mix gently until the batter is smooth and consistent.

Pour the batter into the lined cupcake pan, filling each liner about two-thirds full.

Bake for 18-20 minutes, or until a toothpick inserted in the center comes out clean. Cool in the pan for 5 minutes before transferring to a wire rack.

While cooling, prepare the buttercream: In a mixing bowl, beat the room temperature butter until creamy.

Gradually add the melted chocolate, cocoa powder, and powdered sugar.

Add a pinch of salt and enough heavy cream to achieve a smooth consistency, beating until fluffy.

Once the cupcakes have cooled completely, pipe or spread the buttercream generously on top.