In a large, heavy-bottomed saucepan, combine the whole milk, heavy cream, 1/2 tsp ground nutmeg, 1/2 tsp ground cinnamon, and a pinch of salt. Heat over medium heat, stirring occasionally, until just simmering and small bubbles appear around the edges. Do not boil. Remove from heat.

In a large mixing bowl, whisk the egg yolks and granulated sugar together until pale yellow and slightly thickened, about 2-3 minutes.

Slowly ladle about 1 cup of the warm milk mixture into the egg yolk mixture, whisking constantly to temper the yolks. This prevents the eggs from scrambling.

Gradually pour the tempered egg yolk mixture back into the saucepan with the remaining warm milk, whisking continuously.

Return the saucepan to medium-low heat. Cook, stirring constantly with a heatproof spatula, until the mixture thickens enough to coat the back of a spoon (nappe consistency) and reaches 160°F (71°C) on an instant-read thermometer. This usually takes about 8-12 minutes. Do not boil.

Immediately remove the saucepan from the heat. Stir in the vanilla extract. Pour the eggnog through a fine-mesh sieve into a clean pitcher or large container to remove any cooked egg bits or spices, ensuring a smooth texture.

Let the eggnog cool at room temperature for about 30 minutes, stirring occasionally.

Cover the container and refrigerate for at least 4 hours, or preferably overnight, to allow the flavors to meld and the eggnog to thicken further.

Once thoroughly chilled, stir well. If desired, whisk in the brandy, bourbon, or rum at this stage. Pour into serving glasses and garnish with freshly grated nutmeg.