Preheat your oven to 300°F (148°C) and line a cupcake tray with paper liners.

In a large bowl, sift together the flour, sugar, cocoa powder, baking soda, and salt. Set the dry mixture aside.

In a separate medium bowl, whisk together the egg, milk (or buttermilk), vegetable oil, and vanilla extract until fully combined.

Gradually incorporate the wet ingredients into the dry mixture, stirring until the batter is smooth and free of lumps.

Stir in the hot water. The batter will be quite thin, but that's normal.

Pour the batter into the cupcake liners, filling them halfway.

Bake for 18-23 minutes or until a toothpick inserted in the center comes out with a few moist crumbs attached.

Once baked, let the cupcakes cool in the pan for about 2 minutes, then transfer them to a wire rack to cool completely.

Begin by beating the room-temperature butter in a large mixing bowl until it's light and fluffy.

Slowly add the melted chocolate, blending thoroughly until smooth and

Next, mix in the cocoa powder, ensuring it is well incorporated into the butter mixture.

Gradually add half of the powdered sugar and 2 tablespoons of heavy cream, beating the mixture until smooth and creamy.

Add the remaining powdered sugar along with a pinch of salt, continuing to mix until you reach a smooth consistency.

If needed, adjust the frosting’s texture by adding the remaining heavy cream, one tablespoon at a time, until you achieve the desired consistency for piping.

Using a piping bag fitted with a decorative tip (I recommend Ateco tip 847), frost the cooled cupcakes with the buttercream.

Cupcakes can be kept at room temperature for up to 24 hours. After that, store them in the refrigerator. For the best taste, serve at room temperature within 4-5 days.