In a medium bowl, combine your eggnog with cornstarch. Whisk together until smooth.

Transfer the mixture to a saucepan and place over medium heat. Stir continuously for 5-7 minutes until thickened.

Once thickened, reduce heat to low and add in the butter, stirring until fully incorporated.

Incorporate the vanilla extract and dark rum if desired, stirring well for 2-3 minutes.

If using pectin for thickness, whisk it in now and cook for another 1-2 minutes.

Remove from heat and let the spread cool in the pan for 5-10 minutes.

Pour the slightly cooled spread into sterilized jars while warm and seal tightly.