Make the Pastry Cream: Heat 2 cups of milk in a saucepan over medium heat. In a bowl, whisk together 4 egg yolks, ½ cup sugar, ¼ cup cornstarch, and a pinch of salt. Slowly pour the hot milk into the egg mixture while whisking constantly. Return to the saucepan and cook, stirring until thickened, about 5-8 minutes. Remove from heat, stir in 2 tablespoons butter and 1 teaspoon vanilla extract, then chill for at least 2 hours.

Prepare Choux Pastry: Preheat your oven to 400°F (200°C). In a medium saucepan, combine 1 cup of water, ½ cup of butter, 1 tablespoon of sugar, and a pinch of salt; bring to a boil. Once boiling, reduce heat and add 1 cup flour, stirring until a dough forms. Cook for an additional 2-3 minutes until the dough pulls away from the sides, then let cool slightly before adding 4 eggs one at a time, beating until smooth.

Pipe and Bake: Line a baking sheet with parchment paper. Fill a piping bag with choux pastry. Pipe 4-inch long strips on the parchment, leaving space between them. Bake at 400°F (200°C) for 10 minutes, then reduce to 350°F (175°C) and bake for an additional 15-20 minutes until golden brown.

Cool Éclairs: After baking, turn off the oven and let éclairs sit for 10 minutes in the oven to cool. Transfer to a wire rack to cool completely before filling.

Make Chocolate Glaze: Heat ½ cup heavy cream in a small saucepan until simmering. Remove from heat and add 6 ounces chopped dark chocolate and 2 tablespoons corn syrup. Stir until smooth and let cool slightly.

Assemble Éclairs: Fill cooled choux shells with pastry cream using a piping bag. Dip tops into chocolate glaze and allow excess to drip off. Set aside until glaze has set.