Combine the sugar, cornstarch, nutmeg, cinnamon and salt in bottom of a large saucepan. Stir until there are no lumps of cornstarch. Slowly add the milk while stirring. Heat mixture over medium heat while constantly stirring, until it reaches a simmer. Remove from heat.
Temper the egg yolks by slowly pouring about 1 1/2 cups of the hot milk mixture into the bowl with the egg yolks while whisking. Pour the egg mixture back into the remaining milk mixture in the saucepan. Return to medium heat and continue cooking until slightly thickened. Remove from heat and stir in the vanilla and bourbon, if using.
Pour into a pitcher and refrigerate. Chill for at least four hours, or overnight if possible. Garnish with nutmeg and cinnamon, if desired, and serve.
Notes
*If using unsweetened almond milk, you will want to add more sugar to taste. I added about 2 tablespoons more sugar.Vegan Version: Use almond milk (*see note above) instead of the cow's milk, omit the eggs and tempering process, and increase cornstarch to 4 teaspoons.