Note: Use the very freshest of egg. Consuming raw or undercooked eggs can increase your risk of contracting food-borne illnesses. This rarely happens, but I want you to be aware. If you are worried about food-borne illness, bring the eggs up to a temperature that kills any bacteria. Whisk egg yolks in a bowl until they are the color of lemons. Bring the milk and cream up to a simmer. Whisk about a fourth of a cup of the warm milk into the eggs, gradually adding all of the milk until well blended. Pour the egg mixture into the saucepan and place back over medium heat until thickened. Remove from the heat and add vanilla, and add the liquor if you are serving it spiked. Cover, place in the refrigerator, and chill for at least an hour before serving.