In a medium saucepan, combine sugar, flour, and salt. Then stir in milk. Over medium heat, cook, stirring until mixture thickens and boils, this will take about 10 minutes. Then boil for 1 minute.
Add 6 egg yolks slightly in a bowl and whisk in a small about of the heated milk mixture. Slowly pour the egg mixture back into the milk mixture, stirring constantly while you add the egg mixture
Cook over medium-low heat, stirring until the mixture thickens and coats spoon, about 8 minutes (do not boil). To check the thickness, lift the spoon from the mixture and hold up for 15 seconds - the spoon should not show through the mixture
Remove from heat and stir in vanilla. Strain the filling for a sieve to remove any lumpy cooked egg pieces. Transfer to a blow and cover surface with plastic wrap, and chill for about 2 hours. You can Also make this mixture the night before and store until ready to fill your eclairs
For the Choux Pastry
Preheat oven to 230C/210C fan/450F/gas mark 8. Grease a baking sheet
In a medium saucepan, combine butter and water. Bring to a boil, stirring until butter melts completely. Reduce heat to low, and add flour and salt. Stir vigorously until mixture leaves the sides of the pan and begins to form a stiff ball
Remove from heat, transfer to a separate bowl and allow to cool for 5 minutes
Add eggs, one at a time, beating well to incorporate completely after each addition. The mixture should fall off of a spoon in a V shape. Once this consistency has been achieved, spoon the mixture into a pastry bag fitted with a circular nozzle. Pipe the Chouz Pastry onto the baking sheet in 1 1/2 x 4 inch strips, leaving enough space between each eclair.
Bake for 15 minutes in the preheated oven, then reduce heat to 165C/145C Fan/325F/gas mark 3 and bake 20 minutes more, until crisp on the outside and sound hollow when lightly tapped on the bottom. Do not be tempted to open the oven door sooner than you need to, else this will result in your eclairs hallow shape collapsing
Remove from oven and allow to cool completely on a wire rack
For the chocolate glaze and to assemble
Remove the cooled filling mixture the fridge. Add the double cream to a bowl and whisk on medium speed, until stiff peaks form.
With a rubber spatula, gently fold the whipped cream into filling. Transfer to a piping bag and set aside.
Cut tops off of cooled pastry shells with a sharp knife. Cut the end of the piping bag with the filling and fill shells with the desired amount mixture and replace tops.
Melt the chocolate and butter in a medium saucepan over low heat. Stir in icing sugar and vanilla extract.
Stir in hot water, one tablespoon at a time, until icing is smooth and has reached the desired consistency. Remove from heat, cool slightly, and drizzle over filled eclairs. Refrigerate until serving.