Beat the yolks, granulated sugar, and brown sugar at high speed with a whisk attachment until the mixture is very pale and fluffy and thick and a steady ribbon of the mixture flows down from the whisk when it’s lifted from the bowl, about 3 to 4 minutes. Empty the egg yolk mixture into a blender, and add the milk, cream, tequila, cointreau, nutmeg, and lemon zest. Cover and blend at medium speed for 30 seconds. Stir in any froth on top of the mixture and empty into serving vessel. Garnish with a few pinches of nutmeg and lemon zest. Can be served immediately or chilled. Will keep in an airtight container in the refrigerator for up to 2 weeks.