Preheat your oven to 300°F (148°C). Line a muffin tin with paper liners.

In a large bowl, whisk together the flour, sugar, cocoa powder, baking soda, and salt until evenly combined. Set aside.

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

Pour the wet ingredients into the dry ingredients and stir with a spatula until just combined—no dry streaks remain.

Add the hot water to the batter and mix gently until incorporated. The batter will be very thin; that's normal.

Fill each liner about halfway with batter. Bake for 18–23 minutes, until a toothpick inserted in the center comes out with a few moist crumbs. Do not overbake.

Let the cupcakes cool in the pan for 2 minutes, then transfer them to a wire rack to cool completely before frosting.

In the bowl of a stand mixer fitted with the paddle attachment, beat the softened butter on medium speed until creamy and smooth, about 2 minutes.

Add the melted (but not hot) chocolate and mix on low until fully incorporated, scraping down the sides as needed.

Add the cocoa powder and mix on low until combined, then increase to medium and beat for 30 seconds.

With the mixer on low, gradually add about half of the powdered sugar and 2 tablespoons of heavy cream. Mix until smooth, then scrape the bowl.

Add the remaining powdered sugar and the pinch of salt. Mix on low until incorporated, then increase to medium and beat for 1 minute until fluffy.

If the frosting is too thick, add additional heavy cream 1 tablespoon at a time until you reach a spreadable or pipeable consistency.

Pipe or spread the buttercream onto the cooled cupcakes. (A large star tip like Ateco 847 works beautifully.)

Store frosted cupcakes at room temperature for up to 24 hours; after that, refrigerate. For the best flavor and texture, bring to room temperature before serving. They will keep well for 4–5 days.