Making the choux pastry: Heat water, butter, and salt in a saucepan until the butter melts. Add flour and mix well until the dough comes away from the sides. Transfer the dough to a bowl and add the eggs one at a time, mixing well between each addition. Piping the choux pastry: Transfer the choux pastry into a piping bag fitted with a plain tip and pipe sticks of dough onto a baking sheet lined with parchment paper. Baking the choux pastry: Bake the choux pastry in a preheated oven at 200°C for about 20 to 25 minutes, until golden and crispy. Making the pastry cream: Heat the milk in a saucepan. In a bowl, whisk egg yolks, sugar, flour, and cornstarch. Pour the hot milk over the egg mixture while stirring constantly, then pour the mixture back into the saucepan and cook over low heat until the cream thickens. Adding the filling: Let the éclairs cool and cut them in half. Using a piping bag, fill them with chocolate/vanillia/coffee et ... pastry cream. Making the glaze: Melt dark chocolate and add melted butter. Mix well until smooth. Glazing the éclairs: Dip the top of the éclairs into the chocolate glaze.