Preheat oven to 325 degrees F. Place 4 ramekins or custard cups into a shallow baking dish with hot water on standby to fill the dish with hot water to half-way up the sides of the ramekins.
In a small saucepan, gently heat eggnog over medium heat until hot enough to scald the milk, without boiling, stir or whisk frequently. Small bubbles will appear just along the edge of the pan. Remove hot milk mixture from heat.
Using a mixer, beat the egg yolks in a large bowl well, add sugar if you want eggnog custard sweeter and mix in.
Once the eggnog is heated, gradually add it to the egg yolks while mixing on slow speed until combined.
Pour strained mixture into a pourable container and pour custard into ramekins. Add rum/brandy if desired.
Place filled ramekins in a large baking dish and fill the baking dish with hot water so that it comes 1/2 of the way up the sides of the ramekins, you can also place the ramekins in the baking dish and add the water as well. Be careful not to get any water in the custard!
Bake at 325 degrees F for 40-50 minutes or until the edges are set but the center is still a little jiggly, as it will set as it cools. Baking times will depend on how deep the custard is in the ramekins. (Smaller ramekins for shorter baking time and deeper custard for longer baking time). The top will appear set but underneath it will still jiggle. Refrigerate for 4 hours or overnight up to 3 days.
To serve, sprinkle some freshly grated nutmeg on top if desired or garnish with fresh fruits, whipped cream or shaved chocolate.
If you want creme brulee, see below.