• Preheat oven to 375℉.  Line baking sheets with parchment paper or silpat mat. Set side.
• In a medium saucepan, add water (or milk), butter, sugar, and salt. Bring to a boil over medium heat. Add flour and stir to combine using a wooden spoon or sturdy spatula.  Bring to a boil as you stir. Continue to stir constantly until the dough starts to pull away from the sides of the saucepan.
• Once dough is a smooth, cohesive ball, transfer to the bowl of stand mixer fitted with a paddle attachment. Beat a medium speed until the dough has cooled to just a little warmer than room temperature.  

  *Note: The dough will steam as you beat it.
• Once dough is ready, add the whole eggs one at time. Make sure the egg is incorporated before adding the next one. Scrape down the bowl as needed.  Once all the whole eggs are in, add the egg white and mix to combine. Continue to beat mixture until smooth, thick, and shiny. Use "V-test" to determine if dough is ready for piping.
• Transfer batter to a pipping bag fitted with a 18mm plain round tip or star/fluted tip.  Pipe 4-inch long cylinders on the lined baking sheet, at least 1 inch apart.
• Bake in preheated oven for 20 minutes. Leave eclairs undisturbed during this time. Then, lower oven temperature to 325℉ and bake for another 20-30 minutes until eclairs are dry and feel very light in weight. Transfer to wire rack to let cool.

Pastry Cream:

• Line a half-sheet baking pan with plastic wrap, ensuring the sides of the pan are also covered. Set aside.
• In a large bowl, whisk together cornstarch and about ½ cup of sugar. Add in egg yolks and about ½ cup of milk. Whisk until smooth. Set aside.
• Pour remaining milk into a heavy bottomed sauce pot. Add remaining sugar. Set over medium-high heat. Stir occasionally to ensure the milk does not burn along the sides or bottom of the pan. When the milk has boiled, slowly add about ¼ cup of hot milk into the egg mixture. Whisk constantly to distribute the heat. Continue to pour more milk until a couple tablespoons are left in the saucepan. Then pour the egg-milk mixture back into the saucepan.
• Reduce to medium heat. Switch to a spatula. Stir constantly until the custard has thickened. To cook the cornstarch out, heat the custard until it boils for about 10-15 seconds. Remove from heat. Stir in butter and salt until smooth. Add the vanilla and stir until well combined.
• Pour pastry cream into a plastic wrapped sheet pan. Spread evenly. Cover surface directly with plastic wrap and freeze for 15 minutes* until pastry cream has cooled to room temperature.
• Remove from freezer. Transfer to another container. Cover surface directly with plastic wrap. Keep chilled in the fridge until ready to use.

Assembly:

• Once cooled, pierce three holes on the flat bottom of eclair.  Using a pastry bag fitted with a ⅙-inch or ¼-inch plain round tip, pipe pastry cream into the eclair.

  Note: Alternatively, you can slice the top ⅓ of the eclair lengthwise.  Fill the bottom ⅔ with the pastry cream and replace the cut top.
• Chill filled eclairs in the fridge for 15 minutes before dipping warm chocolate glaze.

Chocolate Glaze:

• Bring heavy cream to a boil. Pour over chopped chocolate and corn syrup.  Let sit for a minute before stirring.  Stir until chocolate is melted and glaze is smooth. Let cool slightly to thicken before using.
• Dip the tops of each eclair into the warm chocolate glaze.  Transfer to a wire rack set over a parchment sheet lined baking sheet to catch any drippings. Chill in the fridge to set the chocolate glaze.