First Step: Preheat Oven and Prepare Ingredients Set your oven to 400°F (200°C) and line baking trays with parchment paper. Brush lightly with water to ensure adequate humidity during baking, which helps the éclairs rise properly.

Second Step: Make the Choux Pastry Dough In a saucepan, combine water, butter, sugar, and salt over medium heat. Once the butter melts and the mixture is boiling, quickly add the flour all at once. Stir vigorously until the dough pulls away from the pan and forms a sticky ball.

Third Step: Incorporate Eggs Remove from heat and let the dough cool slightly for about 10 minutes. Beat in the eggs one at a time, mixing thoroughly after each addition until the dough is smooth, thick, and holds its shape but is not too runny.

Fourth Step: Pipe and Bake Pastry Shells Transfer dough to a piping bag fitted with a large round or star tip. Pipe 4-inch (10-14 cm) logs onto prepared baking sheets, leaving space between each. Smooth any peaks with a damp finger. Brush with egg wash for shine. Bake for 20 minutes at 400°F (200°C), then lower the temperature to 350°F (175°C) and bake for another 8-12 minutes without opening the oven. Cool completely on wire racks.

Fifth Step: Prepare Pastry Cream Whisk egg yolks and cornstarch while heating milk and sugar to a simmer. Temper yolk mixture slowly with hot milk, strain, and cook over medium heat, stirring until thick and bubbling. Remove from heat, stir in butter, vanilla, and salt. Chill for at least 3 hours.

Sixth Step: Make Chocolate Ganache Warm heavy cream with optional corn syrup until just simmering. Pour over chopped chocolate; let sit 1-2 minutes, then stir into a smooth, glossy ganache. Cool until slightly thickened but still dip-able.

Final Step: Fill and Glaze Éclairs Using a piping bag, fill cooled shells with chilled pastry cream through an opening or by slicing horizontally. Dip the tops into ganache and place on trays to set. Ganache will firm at room temperature or can be refrigerated.