1. In a medium saucepan, whisk together the egg yolks, granulated sugar, cornstarch, salt, and a splash of eggnog until smooth and lump free. Slowly whisk in the remaining eggnog.

2. Place the pan over medium heat and cook, whisking constantly, until the mixture thickens, bubbles, and coats the back of a spoon. Remove from the heat and whisk in the butter, vanilla, and nutmeg until glossy.

3. Pour the custard into a shallow dish, press plastic wrap directly onto the surface, and refrigerate until completely chilled, at least 2 hours.

4. To make the choux pastry, combine the water, butter, granulated sugar, and salt in a medium saucepan over medium heat. Bring to a gentle boil, stirring until the butter melts.

5. Add the flour all at once and stir vigorously with a wooden spoon until the dough pulls away from the sides of the pan and forms a smooth ball. Continue cooking and stirring for 1 to 2 minutes to dry the dough slightly.

6. Transfer the hot dough to a mixing bowl and let cool for 5 minutes until warm but not steaming. Beat in the eggs one at a time, mixing well after each addition, until the dough is smooth, thick, and falls from the spatula in a slow V shape.

7. Preheat the oven to 400°F (200°C). Line two baking sheets with parchment paper. Spoon the choux dough into a piping bag fitted with a large round or star tip and pipe 12 to 16 mounds or rosettes, spacing them a couple of inches apart. Gently press down any peaks with a damp fingertip.

8. Bake for 20 to 25 minutes, or until the puffs are deeply golden and feel light when lifted. Turn off the oven, poke a small hole in each puff with a skewer to release steam, and return them to the oven for 5 minutes to dry out. Cool completely on a wire rack.

9. When the custard is cold, whip the heavy cream with powdered sugar in a chilled bowl until medium stiff peaks form. Gently fold about half of the whipped cream into the eggnog custard until smooth and light.

10. Transfer the eggnog cream to a piping bag fitted with a small round or star tip. Slice each puff in half horizontally or poke a small hole in the bottom, then pipe the cream into the centers until generously filled.

11. Arrange the filled cream puffs on a serving platter, dust with powdered sugar and a pinch of extra nutmeg if desired, and serve immediately or chill for up to a few hours before serving.