1. Preheat a heatproof glass or Irish coffee mug by filling it with boiling water for 10 seconds, then discard the water.
2. Add the brown sugar to the glass, pour in the hot coffee, and stir until the sugar fully dissolves.
3. Pour in the Irish whiskey and stir once more.
4. Lightly whip cold heavy cream until it thickens but still flows slowly (do not make stiff peaks).
5. Float the cream: turn a warm spoon upside-down and gently pour the cream over the back of the spoon so it sits on top of the coffee in a distinct white layer.
6. Do not stir again — drink the hot coffee and whiskey through the cool cream.
