Juice the 2 lemons, you need 4-6 Tablespoons of Lemon Juice. (6 Tblsp makes it pretty tart, so you may want to use 4 or a bit more) Grated lemon zest of 1 lemon.

Beat the egg, combine with sugar, lemon juice and lemon zest; cook over low heat until thickened or microwave until thickened. Watch carefully and stir often, so it isn't lumpy. Add butter and stir until butter is melted.
Serve over Gingerbread, pound cake or bread pudding.
Makes about 1 1/2 cups sauce