Making chocolate eclairs is a rewarding process. Follow these steps carefully for the best results:

Preparing the Choux Pastry

1. Preheat your oven to 400°F (200°C). Line two baking sheets with parchment paper.
2. In a saucepan, combine water, butter, and salt. Heat until the butter is completely melted and the mixture comes to a boil.
3. Remove from heat and add flour all at once. Stir vigorously until the dough forms a ball and pulls away from the sides of the pan.
4. Allow the dough to cool for about 5 minutes. Then, add the eggs one at a time, mixing well after each addition until smooth and glossy.
5. Transfer the pastry dough to a piping bag fitted with a large round tip.
6. Pipe 4-inch strips onto the lined baking sheets, leaving space between each eclair.
7. Bake in preheated oven for 25–30 minutes, or until they are golden brown and puffed up. Do not open the oven during baking.
8. Once baked, remove from the oven and cool completely on a wire rack.

Preparing the Chocolate Pastry Cream

1. In a medium saucepan, combine milk and half the sugar. Heat over medium heat until hot but not boiling.
2. In a bowl, whisk together the remaining sugar, cocoa powder, egg yolks, and cornstarch until smooth.
3. Gradually pour the hot milk mixture into the egg yolk mixture, whisking constantly to temper the yolks.
4. Return the mixture to the saucepan and cook over medium heat, stirring constantly until it thickens and reaches a boil.
5. Remove from heat and stir in butter, vanilla extract, and chopped chocolate until smooth.
6. Transfer the pastry cream to a bowl, cover with plastic wrap touching the surface to prevent a skin from forming. Refrigerate until chilled.

Assembling the Eclairs

1. Once the pastry cream is chilled, transfer it to a piping bag with a small round tip.
2. Poke a small hole in the end of each cooled eclair. Fill with pastry cream through the hole until plump.
3. For the chocolate glaze, melt the chocolate chips and butter together in a microwave or double boiler until smooth. Stir until glossy.
4. Dip the top of each filled eclair into the chocolate glaze, allowing excess to drip off.
5. Place the eclairs on a wire rack to set.