Warm your glasses: Fill two Irish coffee glasses or heat-safe mugs with hot water. Let sit for a minute, then discard the water. This keeps your coffee hot longer.
Make the lightly whipped cream: In a cold bowl, whisk the heavy cream (and powdered sugar, if using) until it just begins to thicken and holds soft peaks. You want it pourable but thick enough to float — not stiff.
Build the coffee: Add 1 tablespoon brown sugar to each warmed glass. Pour in the hot coffee, dividing evenly. Stir until the sugar dissolves.
Add the whiskey: Pour 1.5 oz Jameson into each glass. Stir gently to combine.
Float the cream: Hold a spoon just above the surface of the coffee, curved side up. Slowly pour the whipped cream over the back of the spoon so it floats on top. Don't stir!
Serve immediately — sip the hot coffee through the cool cream.