In a medium saucepan, combine ½ cup of unsalted butter, 1 cup of water, and a pinch of salt. Bring the mixture to a rolling boil over medium heat. Once boiling, reduce the heat, quickly add 1 cup of all-purpose flour, and stir vigorously until a dough forms and pulls away from the sides of the pan. Remove from heat and allow it to cool slightly for about 10 minutes.

In a large bowl, transfer the cooled dough and add 4 large eggs, one at a time. Use a hand mixer on medium speed to beat the mixture until it becomes glossy and smooth, about 5 minutes. The dough will be ready when it holds a thick ribbon shape.

Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper. Using a piping bag fitted with a large round tip, pipe small mounds of dough onto the baking sheet, spacing them about 2 inches apart. Bake for 20-25 minutes until the puffs are golden brown and doubled in size. Remove them from the oven and immediately pierce the sides of each puff with a skewer to release steam.

In a mixing bowl, combine 1 cup of heavy cream, 1 cup of eggnog, ¼ cup of sugar, 1 teaspoon of vanilla extract, and ½ teaspoon of freshly grated nutmeg. Using an electric mixer, whip the ingredients on medium-high speed until soft peaks form, around 3-4 minutes. Cover the bowl with plastic wrap and refrigerate for about 30 minutes.

Once the choux pastry has cooled completely, use a serrated knife to cut off the tops of each puff or pierce their bottoms with a small hole. Fill a clean piping bag with the chilled eggnog cream, then pipe it generously into each puff until full.

Before serving, dust the cream puffs lightly with powdered sugar using a fine sieve.