With the heat on medium, stir in the butter, brown sugar, cinnamon, nutmeg, and salt. Return to a boil and continue cooking, stirring occasionally, until the sauce thickens, about 5 minutes. Serve while warm.
2 tablespoons unsalted butter, 1/4 cup brown sugar, 1/4 teaspoon cinnamon, 1/8 teaspoon nutmeg, pinch of salt