Make the pastry cream: in a large bowl, whisk egg yolks, sugar and cornstarch. Whisk until smooth and set aside. Scrape the seeds of ½ vanilla bean. Add the bean and the seeds to a medium saucepan with milk, heat stirring occasionally, until simmer. Remove vanilla bean from the simmering milk. Turn the heat off. Temper the eggs: gradually add the hot milk into the egg yolk mixture, whisking constantly. When combined, pour the mixture back to the saucepan, heat over medium-low heat, stirring frequently, until thickens, about 7-8 minutes. Remove from heat, add butter (if you are using vanilla paste/extract this the time to add it), stir until smooth. Cover with plastic wrap and let cool completely.
Preheat oven to 425F (220C).
Make the choux pastry: place water, milk, butter, sugar and salt to a saucepan. Bring to a boil. Remove from heat and add the flour, stir with a wooden spoon until incorporated. Once flour is incorporated, place back over medium-low heat, stirring constantly for 1-2 minutes or until dough comes together into a smooth ball.
Transfer to a large bowl and let cool slightly. Add the eggs, one at a time, allowing each egg to fully incorporate after each addition. Beat until dough is smooth.
Transfer the dough to a large piping bag with a large ½ inch (1 cm) star tip. Pipe the dough into 4-inch (10cm) long strips on a parchment paper lined baking sheet, keeping them 1½-inch (3-4cm) apart. Dip your finger in water and pat the ends of the eclairs to flatten the pointed ends. Dust each éclair with powder sugar.
Place the baking try in the oven and reduce immediately to 350F (175C). Bake for 18-20 minutes. Quickly prick each of the eclairs with a toothpick.
Transfer to a cooling rack and let cool. With a small pastry tip, poke 2-3 holes through the bottom of each cooled pastry. Pipe the chilled cream inside. Refrigerate eclairs while making chocolate ganache.
Chocolate ganache: Chop the chocolate, place in a large bowl. In a saucepan bring the heavy cream to a simmer. Pour over the chocolate. Let sit for 1-2 minutes. Stir until smooth
Dip the top half of filled and chilled eclairs into the chocolate sauce, allowing excess to drip off.