1. Combine the sugar and egg in a small bowl and beat with an electric mixture until well blended.
2. Stir the melted butter into the egg mixture. Cook in the top of a double boiler over simmering water, whisking constantly, until the sugar has dissolved and the sauce has thickened a bit, about 5 minutes.
3. Remove the pan from the heat and add the lemon juice, nutmeg and bourbon.
4. Serve on rice pudding, bread puddings, pudding cakes or old-fashioned jam pudding. Serve puddings warm with the sauce.