Melt the butter in a heavy saucepan over low heat.
In a small bowl, whisk the egg.
Add the sugar and whiskey to the butter.
Once the sugar has dissolved, slowly temper the egg into the sauce.
Do this by drizzling 1 tablespoon of the liquid over the egg and stirring to combine.
Once you've done this 4 times, slowly pour the mix back into the pot, stirring constantly.
This keeps the egg from getting too hot too quickly and ending up scrambled.
Pour this over bread pudding and serve.
You can also pour 1/2 over the bread pudding and serve the other 1/2 on the side.