Step 1:
In a microwave-safe container (like a 4 cup pyrex measuring cup), combine the sugar, lemon juice, and zest. Add the butter.
Step 2:
Microwave for 30 seconds. Remove from oven, then stir until smooth.
NOTE: Make sure you use SALTED butter. If you don't have any, add a pinch of salt to the icing. This really cuts the sweetness and adds to the complexity of the flavor.