In a medium saucepan, combine the eggnog, granulated sugar, egg yolks, and cornstarch. Use a whisk to mix these ingredients until smooth and well blended.

Place the saucepan over medium heat, stirring constantly with the whisk for 7-9 minutes until the mixture thickens into a pudding-like consistency.

Remove the saucepan from heat, and stir in the ground nutmeg, unsalted butter, vanilla extract, and a pinch of salt.

Pour the mixture through a fine mesh strainer into a bowl or jar to eliminate lumps and ensure smoothness.

Allow the jam to cool for 10-15 minutes at room temperature, then transfer to an airtight jar and refrigerate.