Preheat the oven to 350 degrees F.
To make the choux dough, melt the butter with the water in a saucepan set over medium high heat, and cook until it’s boiling. Add the flour all at once and take the pan off the heat. Once the flour has been stirred in and the mixture turns into a dough, return the pan to the heat and keep stirring the dough with a wooden spoon until the dough pulls away from the sides of the pan and forms into a ball.
Transfer the dough into the bowl of a stand mixer fitted with the whisk attachment. Turn the mixer to medium low and add each egg, one at a time, waiting for each egg to absorb before adding the next one. The dough should be creamy and smooth.
Put the choux dough in a piping bag fitted with a plain tip, and pipe out long eclair shapes, about the size of somewhere between a hot dog and a sausage, onto a parchment paper or silpat lined baking sheet. Bake the eclairs for 30 minutes (they should look golden brown, if not, bake them longer until golden brown on the edges), and immediately after removing them from the oven, poke a hole into the side of each one with a knife, to let the steam escape.
To make the chocolate pastry cream filling, scald the milk in a saucepan set over medium high heat. Scalding means taking the milk to just below a boil, there will be bubbles on the side of the pan, but it’s not quite boiling. In the meantime, whisk the egg yolks with the sugar and cornstarch. Once the milk has scalded, add half of it to the egg yolk mixture while constantly stirring, then add the egg mixture back to the rest of the milk in the saucepan. Return the saucepan to medium heat and whisk the mixture constantly for a couple of minutes, until the mixture thickens up into a pudding-like texture.
Remove the pastry cream from the heat, and let it cool for 10 minutes. In the meantime, melt the chocolate chips in a double boiler (a heatproof bowl set atop a pan with an inch of simmering water). Add the vanilla extract and amaretto to the pastry cream, then stir in the melted chocolate. Whisk until smooth, then chill the pastry cream either in the refrigerator or on an ice bath.
Whip the heavy cream to soft peaks with a whisk or a hand mixer, then stir 1/4 of it into the pastry cream. This will lighten it significantly and make it more manageable. Fold the next 1/4 of the whipped cream into the chocolate pastry cream, and repeat until you use up all of the whipped cream. Place the chocolate filling in a piping bag and pipe the cream into the eclairs. If you have a long enough tip, you can insert the tip into the eclairs and inject the cream all the way inside. If you don’t have a long tip, just cut the eclairs in half along the equator, and pipe the filling on one side.
Melt the bittersweet chocolate over a double boiler, and dip all the top halves of the eclairs into the melted chocolate, then place them onto the pastry cream piped eclair bottoms. It’s now ready to eat. Enjoy!