In a heavy bottom saucepan, combine the 1 cup sugar, 1 cup eggnog, ½ cup salted butter, 1/4 cup bourbon, if using, and 1 teaspoon vanilla extract or bourbon extract .
Heat to a low simmer, stirring with a wooden spoon.
Continue to stir until it thickens and sugar is dissolved (about 5 minutes). Right before removing from the heat, stir in the ¼ teaspoon baking soda, 1/8 teaspoon ground cinnamon and 1/8 teaspoon ground nutmeg until dissolved.
Serve immediately or allow to cool and refrigerate. The syrup will thicken as it cools.
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Notes
Make-Ahead and Storage: This syrup can be made ahead and stored covered in the refrigerator for up to a week. Reheat in the microwave on 50% power in 30 second intervals, stirring between, or in a saucepan over low heat. Use 1-2 tablespoons of milk to thin it out if too thick. Freezer Instructions: This sauce is freezer friendly. Place it into a container (like my favorite deli cups) with just a pinch of air. Air is in the enemy against freezer burn, but it also needs space to expand. Freeze for up to 6 months. Thaw overnight in the refrigerator. Also, freeze in an ice cube tray and take out 1-2 ice cubes when you need some syrup for pancakes or waffles.