In a heavy-bottomed saucepan, combine eggnog and sugar.
Bring to a simmer over medium heat, stirring constantly to dissolve the sugar.
Reduce heat to low and continue to simmer, stirring frequently, for about 20-25 minutes, or until the mixture has thickened to your desired consistency. Keep in mind that it will thicken further as it cools.
Remove from heat and stir in heavy cream, rum extract (if using), and nutmeg.
Pour into sterilized jars, leaving 1/4 inch headspace. Process in a boiling water bath for 10 minutes for proper sealing.
Notes
This jam can be stored in the refrigerator for up to 2 weeks or properly canned for longer storage. Enjoy!