Place a medium metal or glass mixing bowl in the refrigerator to be used later.: The cold bowl helps the heavy cream whip faster, and you will notice a firmer, glossier foam when the bowl is well chilled. It feels cooler to the touch and sometimes you can see faint condensation on the outside. Doing this step early reduces frustration later, because warm bowls make whipping take much longer. Avoid using a plastic bowl here, because it holds residual heat and oils that can impede whipping.
In another medium bowl, beat egg whites with a hand mixer until stiff. Fold in 3/4 cup sugar, a few tablespoons at a time. Set aside.: When you whip the egg whites , listen for a change from watery to a soft swishing sound, and watch them change from glossy foam to peaks that hold shape. Adding sugar gradually helps stabilize the whites and gives them a glossy sheen. If the whites look grainy or collapse, they were either under whipped or contaminated with fat; start with fresh clean utensils. A common mistake is adding all the sugar at once, which can deflate the foam.
In a large mixing bowl, beat egg yolks until they turn a lemon-yellow color. Mix in remaining 1/4 cup sugar a few tablespoons at a time. Then mix in milk, vanilla, nutmeg, and salt. Set aside.: As you beat the egg yolks , they will lighten and become pale, signaling incorporated air and a silkier texture. Gradually beating in the remaining sugar ensures smoothness, while stirring in milk , vanilla extract , nutmeg , and a dash of salt builds layered flavor. The mixture should look cohesive and slightly thickened, not curdled. If you see streaks or lumps, beat a bit longer at low speed. Avoid overheating or rapid mixing which can create a foamy rather than creamy base.
Remove the chilled bowl from the refrigerator. Pour in heavy cream and whip until soft peaks form.: The chilled surface helps the heavy cream transform quickly into soft peaks that gently hold their shape; the sound changes to a thicker, more muffled beat and a ribbon forms when you lift the whisk. The cream should be light and billowy, not overly stiff. If you go too far and form butter, you will see separation and a grainy texture; stop whipping as soon as peaks curl over. If the cream refuses to thicken, it may have been too warm, so re chill the bowl and try again.
Add the egg white mixture and the whipped cream to the egg yolk mixture and use a wooden spoon to stir. Chill until cold, about 2 hours.: When you combine the airy egg whites and fluffy heavy cream with the yolk mixture, the texture becomes luxuriously light yet creamy. Stirring with a wooden spoon gives gentle control, preserving air pockets and producing a silky consistency. You should taste a balanced sweetness and a warm spice note from nutmeg . Chill for at least two hours so the flavors meld and the drink thickens slightly; the aroma will deepen and the surface will become cooler and more cohesive. Avoid vigorous stirring which will knock out air and make the eggnog dense.