Make the Cream Filling: In a medium saucepan, whisk together sugar and cornstarch until combined. Gradually add milk and egg yolks, stirring continuously. Cook over medium heat until thickened, about 7-10 minutes. Remove from heat, whisk in butter and vanilla, cover and refrigerate for at least 2 hours.

Prepare the Choux Pastry: In a saucepan, combine water, butter, and salt over medium heat. Bring to a boil, ensuring butter melts. Add sifted flour and stir until a smooth ball forms, about 3-5 minutes. Remove from heat and cool slightly.

Incorporate the Eggs: Beat in one egg at a time into the slightly cooled dough until smooth and glossy, about 5-7 minutes of mixing.

Pipe the Eclairs: Preheat oven to 400°F and line baking sheets. Transfer pastry dough to a piping bag and pipe 4-inch strips onto sheets, spacing 2 inches apart.

Bake the Eclairs: Bake for 15 minutes. Reduce temperature to 350°F without opening the door and bake for another 20 minutes until golden brown. Cool completely on wire rack.

Fill the Eclairs: Poke a hole in the underside of each eclair and fill with chilled pastry cream using a piping bag.

Make the Chocolate Glaze: In a microwave-safe bowl, combine chocolate and heavy cream. Heat in 30-second intervals until melted and smooth.

Glaze the Eclairs: Dip the tops of each filled eclair into the glaze and let set for about 20 minutes.