Combine 4 cups hot coffee, 1/4 cup sugar, and 1/2 teaspoon lemon zest in a large mixing bowl. Coffee should be hot to ensure that sugar dissolves. Whisk ingredients together until well combined.
Pour coffee mixture into a medium baking sheet or brownie pan. A pan with raised sides will help prevent spills. Place in freezer for 30 minutes, or until crystals begin to form.
Repeat this process every 15-20 minutes, continually breaking up the crystals that forms as your mixture freezes. As you do this, you'll see that your mixture takes on a slushy, almost snow-like consistency.