1. In a medium saucepan, combine 1/2 cup butter, 1 cup water, and 1/4 teaspoon salt over medium heat. Bring to a boil.

2. Remove from heat and quickly stir in 1 cup flour. Return to heat and stir vigorously until a smooth dough forms and pulls away from the sides, about 1 minute.

3. Transfer the dough to a bowl and cool for 5 minutes. Beat in the eggs one at a time, fully incorporating each before adding the next.

4. Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper.

5. Pipe or spoon dough into 1.5-inch mounds, spacing them apart. Smooth any peaks with a wet fingertip.

6. Bake for 20–25 minutes until puffed and golden. Don’t open the oven door while baking.

7. For the filling, heat 2 cups milk and 1/2 teaspoon nutmeg in a saucepan until steaming.

8. In a bowl, whisk together 4 egg yolks, 1/2 cup sugar, and 3 tablespoons cornstarch until pale.

9. Slowly pour the hot milk into the yolk mixture while whisking. Return everything to the saucepan and cook until thickened.

10. Remove from heat, stir in 1 teaspoon vanilla extract, and cool completely.

11. Whip 1/2 cup heavy cream to stiff peaks, then fold it into the cooled custard.

12. Cut puffs in half, fill the bottoms with eggnog cream, and replace tops. Dust with powdered sugar before serving if desired.