Whip the Yolks & Sweetener: In a large mixing bowl, beat the egg yolks with a hand mixer until they become light yellow and creamy. Gradually add the powdered Besti sweetener, continuing to beat until it's fully dissolved. Set this mixture aside.
Warm the Milk & Spices: Pour the almond milk, heavy cream, cinnamon, and nutmeg into a medium saucepan. Gently heat the mixture, stirring occasionally, until it just starts to bubble around the edges. Take it off the heat immediately and stir in the vanilla extract.
Temper the Eggs: This crucial step prevents scrambling! Slowly pour a thin stream of the warm milk mixture into the egg yolk mixture while continuously whisking. Keep whisking to gradually bring the egg yolks up to temperature.
Thicken the Eggnog: Pour the combined mixture back into the saucepan. Cook over medium-low heat, whisking constantly, until it thickens slightly or reaches about 160°F (71°C). Do not let it boil! It will continue to thicken as it cools.
Add Bourbon (If Using): If you're adding bourbon, stir it into the warm eggnog now. This allows its flavor to meld beautifully.
Chill and Enjoy: Transfer your sugar-free eggnog to a bowl, bottle, or pitcher, cover it, and refrigerate for at least 3-4 hours until it's thoroughly chilled and thick. Serve cold, perhaps with a dollop of sugar-free whipped cream and an extra sprinkle of cinnamon or nutmeg!