For Choux Paste: *This recipe makes enough for half the amount of choux paste above. For a second batch of vanilla pastry cream, omit the coffee powder and stir in 1/4 teaspoon vanilla after removing the thickened pastry cream from the stove.
For Chocolate Glaze for Eclairs: *This is enough to frost 24 éclairs of this size, so cut the recipe in half if you only plan to make 12, as we did here; or make the whole recipe and drizzle the remainder over ice cream.
Choux Paste: Preheat the oven to 450°F. Combine the water, butter and salt in a saucepan, heat till the butter has melted, and then bring the mixture to a boil. Remove the pan from the heat and add the flour all at once. Beat vigorously with a large spoon until the mixture leaves the sides of the pan and forms a cohesive ball around your spoon. Cool slightly.
Add the eggs, one at a time, beating after each until the mixture is glossy and smooth. You may use an electric mixer to do this, or you may use a wooden spoon and give yourself a workout. For cream puffs, drop the mixture by rounded tablespoonfuls (or use the large round tip of a pastry bag to form 1 1/2-inch rounds) on a waxed paper or parchment-lined baking sheet, leaving a bit of space between them. For éclairs, use the large round tip of a pastry bag to shape 3 1/2 x 1-inch fingers on the baking sheet, also leaving a bit of space between them. (We split the dough and shaped half of it into cream puffs and the other half into éclairs; after all, variety is the spice of life.)
Bake for 15 minutes, then lower the heat to 350°F and bake until the sides are stiff, about 20 minutes more. Make a small slit in the end of all the pastries and return them to the oven for 5 minutes for the steam to escape. Cool on racks while you prepare the pastry cream.
Pastry Cream: Sift the sugar, flour and cornstarch together in a medium bowl. Add the egg and beat until the mixture is light.
In a medium-sized saucepan set over medium heat, combine the milk and instant coffee (if you're using it), stirring to dissolve. Bring the milk to a boil. Gradually add half of it to the egg mixture, whisking rapidly all the while. Pour the entire mixture back into the pan and cook over high heat, stirring constantly with your whisk, until the mixture is bubbling and very thick. Keep whisking continuously, back and forth and around the edges of the pan. Remove from the stove.
Stir in the vanilla (if you're using it). Pour the cream into a glass or ceramic bowl, place plastic wrap directly over the surface of the cream to prevent a skin from forming, and refrigerate. When the pastry cream has cooled, cut each cream puff or éclair in half, fill the bottom halves with cream, and replace the top halves.
Eclair Glaze: Melt the butter with the chocolate in a microwave oven or in the top of a double boiler set over simmering water. Remove from the heat. Stir in the water and sugar. Beat with a spoon or fork until the glaze is of a smooth but not stiff consistency, and drizzle it onto the éclairs.