For the pastry cream:
3/4 cup sugar
A pinch of salt
3 1/4 cups milk, divided
1/4 cup cornstarch
3 egg yolks
1 1/2 teaspoons vanilla extract
For the éclair dough:
1 cup water
6 tablespoons (3/4 stick) butter
1/2 teaspoon salt
1 cup flour
For the topping:
1 recipe Simple Chocolate Frosting or 8 ounces semi-sweet chocolate, melted
Make the pastry cream: In a medium saucepot over medium heat, combine the sugar, salt and 3 cups milk, and bring up to a bubble. While the milk is heating up, combine the remaining 1/4 cup of milk with the cornstarch in a bowl and stir until completely dissolved. In another large bowl, break up the egg yolks and whisk them together.
Using a whisk, add the cornstarch mixture into the simmering milk and stir continuously until thickened, about 2 minutes (it's ok if the mixture boils lightly). When the mixture has thickened up, slowly add a couple ladlefuls of the hot liquid into the bowl with the egg yolks while whisking constantly. Add the egg yolk mixture back to the pot, return it to medium heat and simmer until thickened, about 2 minutes. Remove the pot from the heat and stir in the vanilla extract.
Transfer the mixture to a bowl and cover with plastic wrap, pushing the plastic down so that it covers the surface of the cream. Refrigerate until chilled, at least 2 hours and up to overnight.
Make the pastry:
Pre-heat oven to 400ºF.
In a medium saucepot over medium-high heat, combine water, butter, and salt. Heat until butter has melted and water is boiling. Add flour and cook, stirring constantly with a wooden spoon. Stir until mixture begins to create a dough ball in the center of the pot and dough is completely pulling away from the sides of the pan, about 2 minutes. Transfer mixture to a mixing bowl or bowl of a stand mixer fitted with a paddle attachment. On low speed, add the eggs to the mixture one at a time, scraping the sides of the bowl well after each addition and beating until the bowl feels cool (the mixture should be very smooth and silky). Transfer the mixture to a plastic food storage bag and cut a half-inch off of one corner to create a pastry bag. On a baking sheet lined with parchment paper, squeeze the mixture into desired shapes and bake until golden brown and puffed, about 40 minutes.
The pastries can be made a day or two ahead of time and kept in an airtight container at room temperature. If they feel soggy when you take them out, pop them into a 400ºF oven for a couple of minutes until they crisp up again. Allow them to cool before filling.
To assemble éclairs (they should be assembled the day of serving): Give the pastry cream a stir to break it up and smooth it out then transfer it to a plastic food storage bag. Cut off about a quarter-inch from one corner to make a pastry bag. Using a chopstick or the tip of a pair of scissors, poke a hole in one end of each baked pastry. Squeeze the pastry cream into each opening, filling the pastries. To finish, dip the top half of the éclairs in melted dark chocolate and refrigerate them to set the chocolate. Or spread a Simple Chocolate Frosting on top instead of the melted chocolate.