Preheat a glass or mug with hot water, then discard the water
Add 1 to 2 teaspoons of brown sugar to the glass
Pour in 1 1/2 ounces of hot Irish whiskey
Stir until the sugar dissolves
Add 4 to 6 ounces of hot brewed coffee
Stir gently to combine
Lightly whip 1 to 2 ounces of heavy cream until just thickened
Pour the cream over the back of a spoon onto the coffee so it floats on top
Serve immediately without stirring