1. In a medium saucepan, heat the milk and heavy cream over medium-low heat. Bring to a gentle simmer, then remove from heat.
2. In a mixing bowl, whisk the egg yolks and sugar until thick and creamy.
3. Slowly pour a small amount of the hot milk mixture into the egg yolk mixture, whisking constantly to temper the eggs and prevent curdling.
4. Pour the egg mixture back into the saucepan with the rest of the milk. Cook over low heat, stirring constantly until the mixture thickens slightly and coats the back of a spoon (about 5 minutes).
5. Remove from heat and add the chocolate chips, stirring until fully melted and smooth. Stir in vanilla extract, cinnamon, nutmeg, and cloves.
6. If desired, stir in the rum or bourbon for a spiked version.