Brown sugar and toasted nuts make a superb filling.
Combine in a small saucepan:
      1 cup packed light brown sugar
      4 tablespoons unsalted butter, cut into small pieces
      2 tablespoons water
      1/2 teaspoon salt
Cook, whisking, over low heat until the butter is melted and the mixture begins to simmer. Remove from the heat and whisk in:
      2 large egg yolks
Return to low heat and cook, whisking constantly, until thickened, 1 to 2 minutes. Remove from the heat and stir in:
      1 1/2 cups chopped walnuts or pecans, toasted
Let cool before using. This keeps, refrigerated, for about 1 week. Soften before using.