Fill a heatproof glass or mug with hot water and let it sit for 1 minute to warm, then pour the water out.

Add the brown sugar and Irish whiskey to the warm glass and stir until the sugar mostly dissolves.

Pour in the hot coffee, filling the glass to about 1 inch from the top, and stir once.

Whip the cream just until it thickens slightly but still pours, it should move like heavy syrup, not hold stiff peaks.

Pour the cream gently over the back of a spoon held just above the surface of the coffee, so it layers on top instead of mixing in.

Serve immediately, while the cream still sits in a distinct layer and the coffee underneath is hot.