To start, preheat your oven to 350°F (175°C) and line a 12-cup muffin tin with cupcake liners. In a large bowl, sift together the sugar, flour, cocoa powder, baking powder, baking soda, and salt. Whisk the dry ingredients well to combine.

In a separate bowl, whisk the egg, milk, vegetable oil, and vanilla extract until smooth. Stir in the brewed espresso. Pour the wet mixture into the dry ingredients and stir until just combined. Be careful not to overmix.

Next, add the boiling water slowly while stirring. The batter will be thin—this is normal and helps create a moist crumb. Once the batter is smooth, divide it evenly between the cupcake liners, filling each about ⅔ full.

Bake for 18–22 minutes, or until a toothpick inserted in the center comes out clean. Let the cupcakes cool in the pan for 5 minutes, then transfer them to a wire rack to cool completely before frosting.

To make the buttercream, beat the softened butter until light and fluffy, about 2–3 minutes. Gradually add the powdered sugar, mixing on low at first to avoid a cloud of sugar. Once incorporated, add the espresso, vanilla, and a pinch of salt. Beat for another 2–3 minutes until smooth and creamy. If you’d like a mocha version, mix in cocoa powder at this stage.

Use a piping bag to swirl the frosting on cooled cupcakes, or spread it with a knife for a more rustic look. Finish with your choice of toppings and enjoy.