Preheat the oven to 350°F (175°C) and line a 12-cup muffin pan with paper liners.

In a large bowl, whisk together the flour, brown sugar, granulated sugar, cocoa powder, baking powder, baking soda, and salt until well combined.

In a separate bowl, mix the egg, milk, oil, and vanilla until fully blended.

Pour the wet mixture into the dry mixture and stir until no dry pockets remain.

Dissolve the instant espresso powder in the boiling water and whisk into the batter until silky and glossy.

Divide the batter evenly among the muffin liners, filling each about two-thirds full.

Bake for 15-18 minutes, or until a toothpick inserted comes out clean. Cool in the pan for 15 minutes before transferring to a wire rack.

Beat the softened salted butter until creamy, then mix in cocoa powder, vanilla, and 2 cups of powdered sugar on low speed.

Gradually mix in remaining powdered sugar and add milk as needed to achieve a thick, spreadable consistency.

Frost cooled cupcakes generously.