Spoon batter into a warmed mug with handle. Stir in fernet and cognac, then hot milk. Grate nutmeg on top.
For the batter: Separate 3 eggs into two bowls. Beat the whites with 1/8 tsp. cream of tartar until soft peaks form. Beat the yolks with 1/2 oz. aged rum, then beat in 1 c. superfine sugar, and ground spices (1/8 tsp. each cinnamon, mace, and allspice plus a small pinch of cloves). Fold in the beaten egg whites.