Heat the oven to 350°F. Line an 8x5 loaf ban with parchment so that the extra hangs over the sides. Coat the parchment and exposed sides of the pan with nonstick cooking spray.

In a standing mixer with a whisk attachment or with a hand mixer, whisk the eggnog at high speed until it doubles in volume, 6-8 minutes. Because of the eggs and other ingredients, it won't actually form peaks like whipped cream, but it should become very frothy and airy. Keep the whipped eggnog in the refrigerator until ready to use.

In a small mixing bowl, whisk together the flour, baking powder, salt, and nutmeg. Set aside.

In a standing mixer with a paddle attachment or with a hand mixer, beat the butter at medium speed until it become creamy, 1 to 2 minutes. Scrape down the sides of the bowl and beat in the sugar until the mixture is looks fluffy and light-colored, 3 to 5 minutes. Beat in the eggs and egg yolk one at a time, fully incorporating each egg before adding the next. Beat in the vanilla and bourbon. Scrape down the sides as needed.

Sprinkle the flour mixture over the butter mixture. Mix on low speed just until the ingredients come together into a dough. It's OK if there is still a little flour visible on the dough and the sides of the bowl.

Using a spatula, gently mix 1/3 of the whipped eggnog into the batter. Pour the rest of the eggnog into the bowl and gently fold it into the batter. At first, it will look like the eggnog won't go into the batter. Just keep gently folding and stirring, and eventually it will form a smooth, glossy, pourable batter.

Pour the batter into the prepared loaf pan and smooth the top. Place in the oven and bake for 50 to 60 minutes. The loaf is done when the top is domed and golden-brown, and when a toothpick inserted into the middle comes out clean.