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You can make some using a coffee maker or one part instant coffee and one part hot water. You can also use a shot of espresso. This will break up any clumps that might be in the sugar, resulting in a smoother texture when you finish. Pour the coffee into the sugar while mixing the sugar briskly with a fork or a small whisk. You will get a thick, syrup-like consistency. Work quickly, as this glaze will harden up.

Summary:
Prepare the strong-brewed coffee and set it aside. Pour the powdered sugar through a sieve into a bowl. Mix in the strongly-brewed coffee. Pour the glaze over your cake, cupcake, or pastry.