Brown sugar and toasted nuts make a superb filling.
Combine in a small saucepan:
Cook over low heat, whisking, until the butter is melted and the mixture begins to simmer. Remove from the heat. Whisk in a small bowl:
Add a little of the hot brown sugar mixture to the yolks to warm them, whisking constantly, then whisk the tempered yolks into the saucepan. Return to low heat and cook, whisking constantly, until thickened, 1 to 2 minutes. Remove from the heat and stir in:
Use immediately.