Slowly heat half & half, heavy cream, brown sugar, cinnamon and vanilla until sugar has dissolved. DO NOT BOIL.
Beat egg yolks in separate bowl until fluffy. Slowly add milk mixture to eggs while constantly stirring.
Pour egg and milk mixture back into saucepan and cook until it slightly thickens enough to coat the back of a spoon. DO NOT BOIL since boiling will cause the mixture to curdle.
Chill for several hours
Pour Amaretto and Allspice Dram in a cocktail shaker with 6 ounces of the eggnog mixture and shake with 3–4 cubes of ice.
Pour into coffee cup & dust with freshly grated nutmeg.