Make Pastry Cream:

1. Whisk egg yolks and cornstarch in a heatproof bowl.
2. Heat milk and sugar in a saucepan until simmering.
3. Temper egg yolks by slowly pouring hot milk mixture into them, whisking constantly.
4. Pour mixture back into saucepan and cook over medium heat, whisking constantly, until thickened.
5. Remove from heat and whisk in butter, vanilla, vanilla bean seeds, and salt.
6. Refrigerate for at least 3 hours.

Make Choux Pastry:

1. Prepare choux pastry dough.
2. Preheat oven to 400°F (204°C). Line baking sheets with parchment paper and brush with water.
3. Pipe 4-inch logs onto baking sheets. Brush with egg wash.
4. Bake for 20 minutes, then reduce heat to 350°F (177°C) and bake for 8-12 more minutes.
5. Cool completely.
6. Make Ganache:
7. Place chopped chocolate in a bowl.
8. Heat cream until simmering and pour over chocolate.
9. Stir until chocolate is melted and smooth.
10. Cool for 15 minutes.

Fill Eclairs:

1. Transfer pastry cream to a piping bag.
2. Prick a hole in each eclair and pipe in pastry cream.

Top Eclairs:

1. Dip the tops of eclairs in chocolate ganache.
2. Let ganache set before serving.