Preheat oven to 300°F (148°C) and prepare a cupcake pan with liners.

Add the flour, sugar, cocoa powder, baking soda, and salt to a large mixing bowl and combine. Set aside.

Add the egg, milk, vegetable oil, and vanilla extract to another medium-sized bowl and whisk together to combine.

Add the wet ingredients to the dry ingredients and mix until well combined.

Add the water to the batter and mix until well combined. The batter will be very thin.

Fill the cupcake liners about halfway and bake for 18-23 minutes, or until a toothpick comes out with a few moist crumbs.

Remove the cupcakes from the oven and allow to cool for 2 minutes, then remove to a cooling rack to finish cooling.

To make the frosting, beat the butter in a large mixer bowl until smooth.

Add the melted chocolate and mix until well combined and smooth.

Add the cocoa powder and mix until well combined and smooth.

Add about half of the powdered sugar and 2 tablespoons of heavy cream and mix until well combined and smooth.

Add remaining powdered sugar and salt and mix until well combined and smooth.

Add remaining heavy cream as needed to get the right consistency of frosting.

Pipe the frosting onto the cupcakes.

Store cupcakes at room temperature for up to 24 hours, then refrigerate. Cupcakes are best served at room temperature. Best eaten within 4-5 days.