Place sugar and milk in the top of a double boiler. Simmer until the sugar is dissolved and mixture is well blended and creamy. Stir in the butter and lemon zest.
Pour over fresh baked gingerbread while still warm.
Notes
This will keep several weeks in the refrigerator.
If you do not have a double boiler, place a heatproof bowl over a saucepan of simmering water.