1. Combine iced coffee, 2 tablespoons of the liqueur, and lemon zest in an 8-inch square baking dish. Cover with plastic wrap and place in freezer on a level surface.
2. After about 20 minutes, scrape ice crystals with a fork. Repeat about every 20 minutes until all of the mixture is frozen into a pile of flaky ice crystals. Allow mixture to sit in freezer for an additional 15 minutes to firm up before serving.
3. Divide among 4-6 glasses and top with whipped cream and a dusting of cocoa powder for garnish.

for the Whipped Cream

1. Place heavy cream, sugar, vanilla extract, and 1 tablespoon of white chocolate liqueur into a mixing bowl and whisk (I use my stand mixer... and electric hand mixer also does the trick) just until the cream reaches stiff peaks.