To make the pastry cream , whisk together the sugar , corn flour and salt in a saucepan. In another bowl, whisk together the milk and egg yolks until smooth. Pour the yolk -milk mixture into the sugar-cornflour mixture while whisking continuously. Add the butter and heat on medium heat while whisking continuously. Once the butter melts and mixture boils, lower flame and cook until custard thickens while stirring. Add vanilla , pass through a sieve into a clean bowl. Cover with cling wrap so that it touches the surface of the cream. Chill for atleast 2 hours.
To make the choux pastry , preheat your oven to 200 C/ 400 F. Line a baking sheet with parchment or grease well. In a medium saucepan, heat the water and the butter. When the butter melts and it boils, stir in the sugar and salt. Add the flour and continue heating while stirring until a film appears on the bottom of the pan. This will take 2 to 3 minutes . Let cool for 3 to 5 minutes. Add the eggs one by one , mixing really well after each. The dough should be thick and hold a peak. Fill into a piping bag with a wide round tip or cut 1/4 inch off the tip of the bag. Pipe lines of around 3 to 4 cms about 4 cms apart. Bake at 400f or 200 c for 20 minutes , and then at 180 C or 350 F for 15 minutes until they are golden brown. Remove from oven, poke a hole into each. Cool on a wire rack.
To make the ganache,chop the chocolate into small pieces. Heat the cream until it bubbles around the edges. Pour over chopped chocolate and stir until smooth.
To assemble , fill the pastry cream in a piping bag with a small tip. Poke a hole in one or both ends of the shell. Insert the tip and fill. Dip the tops of the filled eclairs in the ganache , shake off excess and let dry.