1. Pastry cream: In a medium saucepan, heat the milk with 1/4 cup of the sugar over medium heat until steaming and small bubbles form at the edge. Do not let it boil over.

2. Meanwhile, in a medium bowl, whisk the egg yolks, the remaining 1/4 cup sugar, the cornstarch and salt until smooth and pale.

3. Temper the yolks: slowly pour about half of the hot milk into the yolk mixture while whisking constantly. Pour everything back into the saucepan.

4. Cook over medium heat, whisking constantly and reaching into the corners, until the cream thickens and large bubbles break the surface. Keep it bubbling and keep whisking for 1 full minute so the cornstarch fully thickens.

5. Off the heat, whisk in the butter and vanilla. Strain through a fine-mesh sieve into a clean bowl. Press plastic wrap directly on the surface and refrigerate until cold, at least 2 hours (up to 3 days).

6. Pastry cups: While the cream chills, grease a standard 12-cup muffin tin. On a lightly floured surface, roll each puff pastry sheet into a 12 x 8-inch rectangle and cut it into six 4-inch squares (12 total).

7. Press each square into a cup so the center lies flat on the bottom and the four corners stand up. Prick the bottoms several times with a fork. Brush the corners with egg wash, if using. Chill the tin for 15 minutes.

8. Heat the oven to 400°F (200°C). Bake for 15 to 18 minutes, until deep golden and crisp. While the shells are hot, gently press the puffed centers down with the back of a spoon to reopen the wells. Cool in the tin for 5 minutes, then cool completely on a rack, about 25 minutes.

9. To serve: whisk the cold pastry cream until smooth. Pipe or spoon about 3 tablespoons into each cooled shell. Dust with nutmeg and a pinch of cinnamon. Serve within 2 hours, or refrigerate.