Make the chocolate filling: In a bowl whisk egg yolks, sugar, and cornstarch to combine. In a heavy-bottomed saucepan bring milk over medium-high heat to a simmer. Cook for 2 minutes, stirring constantly. Stir about ½ cup of hot milk into the egg yolks to slowly warm it. Then pour warmed egg mixture slowly into the saucepan, whisking constantly. Cook until filling is thick, for about 1-2 minutes. Whisk continuously.
Remove from heat and stir in chocolate and salt until smooth and fully combined for about 1-2 minutes. Pour through a mesh strainer into a bowl and let cool for about 5-10 minutes. Stir occasionally. Add butter and stir to combine. Cover with plastic wrap to prevent forming a skin. Refrigerate at least 1-2 hours or overnight.
Preheat oven to 375°F / 190°C. Line 2 baking sheets with parchment paper. Set aside.
Make the Pâte à Choux: In a heavy-bottomed saucepan bring water, butter, sugar, and salt to a boil. With a wooden spoon stir in flour and cook for about 2-3 minutes until it starts to form a film on the bottom of the pan. Transfer to a large mixing bowl and let cool about 2 minutes.
Stir in one egg at the time. After every egg, the dough will separate. Keep stirring until the dough comes together again. In the end, the dough will be sticky and firm enough to hold a stiff peak.
Make the egg wash: In a small cup stir egg and water to combine. Set aside.
Transfer Pâte à Choux to a piping bag fitted with a star tip (I used the Wilton 1M tip). Stick a bit of dough on the baking sheets underneath the parchment paper to fix the paper. Pipe ten about 4-inch long and ½-inch wide (10x1,5cm) strips on the 2 prepared baking sheets with enough space in-between (about 2-3 inches) and spin the tip. Press down the tip with wet fingers that they don't get too brown or burn. Lightly brush with egg wash.
Bake one baking sheet at the time for about 30-35 minutes until puffed and golden brown. They need to sound hollow when knocking on the bottom. Don't open the oven during the baking time or the eclairs will collapse. I baked mine for 33 minutes. Let cool to room temperature.
Fill the cream puffs by cutting the eclairs in half horizontally. Pipe the chocolate filling into the eclairs. Set aside.
Make the chocolate ganache: Place chopped chocolate in a large heatproof bowl. Microwave heavy whipping cream until very hot and lightly simmering for about 1-2 minutes. Pour over chocolate and let stand 1-2 minutes. Stir until smooth, and the chocolate is completely melted. Add corn syrup and stir until combined if desired (for extra gloss, totally optional). Let stand for about 5 minutes.
Place eclairs on a wire rack. Spoon chocolate ganache (about 1 tablespoon of ganache per eclair) on top of the eclairs and let dry. Store leftovers in an airtight container in the fridge up to 3 days.