Preheat and prep. Heat oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners.
Mix dry ingredients. In a large bowl, whisk flour, black cocoa, sugar, baking soda, baking powder, and salt until evenly combined.
Whisk wet ingredients. In a separate bowl, whisk buttermilk, oil, eggs, vanilla, and espresso powder.
Stir in the hot water last. If you want a deeper black, add a small squeeze of black gel food coloring now.
Combine gently. Pour wet ingredients into dry. Whisk or fold just until smooth.
The batter will be thin—that’s good for a velvety crumb.
Fill liners. Divide batter evenly among the liners, filling each about two-thirds full. Use a scoop for tidy portions.
Bake. Bake 16–20 minutes, until a toothpick comes out with a few moist crumbs. Don’t overbake.
Cool completely. Let cupcakes cool in the tin for 5 minutes, then transfer to a rack.
Cool fully before frosting.
Make the frosting. Beat softened cream cheese and butter together until smooth and fluffy, 2–3 minutes. Add vanilla and salt. Gradually beat in powdered sugar until creamy and pipeable.
Tint with black gel or charcoal powder if you want a spooky hue.
Frost and decorate. Swirl frosting onto cooled cupcakes with a spatula or piping bag. Add sprinkles or candy eyes for a Halloween touch.