Choux Pastry: Preheat oven to 400 degrees F (200 degrees C) and place rack in center of oven. To make all the eclairs  the same size, I use a template. Take a piece of parchment paper and draw 12 - 3 1/2 inch (8.5 cm) lines, spacing the lines about 2 - 3 inches (5 - 7 cm) apart. Place the template under your parchment paper so you can use it as a guide.

In a bowl whisk the flour with the sugar and salt. Place the butter and water in a saucepan over medium high heat and bring to a boil. Remove from heat and, with a wooden spoon, add the flour mixture, all at once, and stir until combined. Return saucepan to the heat and stir constantly until the dough comes away from the sides of the pan and forms a thick smooth ball (about 1-2 minutes). Transfer the dough to your electric mixer (can do this by hand or with a hand mixer), and beat on low speed to release the steam from the dough (about 1 minute). Once the dough is lukewarm start adding the lightly beaten eggs (dough will separate and then come together) and continue to mix until you have a smooth thick paste (dough will fall from a spoon in a thick ribbon). Place the dough in a pastry bag fitted with a large plain tip and pipe 12 oblongs of dough (about 3/4 inch (2 cm) wide) onto the baking sheet (using template as a guide). (When piping have the bag at a 45 degree angle.) If desired, with a pastry brush, gently brush the tops of the dough with the lightly beaten egg. 

Bake for 15 minutes and then reduce the oven temperature to 350 degrees F (180 degrees C). Continue to bake for a further 25 minutes or until the shells are a nice amber color and when split, are almost dry inside. Remove from oven and place on a wire rack to cool.

Pastry Cream: In a heatproof bowl, mix the sugar and egg yolks together. Whisk the flour with the cornstarch and then add to the egg mixture, mixing until you get a smooth paste.

Meanwhile, in a saucepan bring the milk just to a boil (just until milk starts to foam up.) Remove from heat and add slowly to egg mixture, whisking constantly to prevent curdling. (If necessary pour through a strainer.) Then pour the egg mixture into a medium saucepan and cook over medium heat until boiling, whisking constantly. When it boils, keep whisking constantly for another 30 - 60 seconds until the pastry cream becomes thick. Remove from heat and stir in vanilla extract. (For a chocolate pastry cream stir in 2 ounces (55 grams) finely chopped semi sweet chocolate. For a mocha flavor add 1 1/2 teaspoons instant coffee or espresso powder to the hot milk.) Pour the pastry cream into a clean bowl and immediately cover the surface with plastic wrap to prevent a crust from forming. Cool to room temperature then refrigerate until firm (can be made up to 3 days ahead). Whisk or stir before using to get rid of any lumps that may have formed.

Chocolate Glaze: Place the chocolate in a heatproof bowl. Heat the cream just until boiling and immediately pour it over the chocolate. Gently stir until the chocolate has melted. Stir in the vanilla extract and corn syrup.

To Assemble: Split the pastry shells in half, lengthwise. Take the top shell and dip into the chocolate glaze, letting the excess drip off. Place on a wire rack to dry. Fill the bottom half (can spoon or pipe) with the cream. Once the glaze is dry, gently place the top half of the pastry shell on the cream. Can be covered and stored in the refrigerator for two days.

Makes 12 Chocolate Eclairs.