This recipe can also make 8 chocolate éclairs, if you pipe, bake, and fill long strips of batter rather than dollops; or gougères, if you beat 1 1⁄2 cups Gruyère or cheddar cheese into the batter after adding the eggs. Puffs may be made 2 days ahead, kept in an airtight container, re-crisped in a 400°F oven 5 minutes, then cooled before filling.
In a heavy saucepan over high heat, bring to a boil 1 1⁄4 cups water, the butter, and salt. Lower the heat to medium. Add the flour, remove from the heat, stir vigorously to combine, then return to the heat, stirring, for 1 minute.
Transfer the batter (a.k.a. pâte à choux) to the bowl of a stand mixer fitted with the whisk attachment. Let cool for 10 minutes. Beat in the eggs, one at a time, on high speed, until combined after each addition. Don’t worry if the batter is clumpy, like scrambled eggs, at first; it will come together. After the final egg has been combined, beat for 2 minutes more.
Preheat the oven to 425°F. Line two baking sheets with parchment paper.
Using a #40-size cookie scoop or a tablespoon, scoop about 24 (1 1⁄2 tablespoon) mounds onto the prepared baking sheets, leaving 1 1⁄2 inches between mounds. Bake in upper third of the oven for 10 minutes. Reduce the temperature to 400°F and bake 20 minutes more. Remove from the oven. Using a skewer, poke 1 (1⁄4-inch) hole into the bottom of each puff to release steam and help retain crispness, then return the puffs to the turned-off oven for 30 minutes. Split each puff in half to make top and bottom pieces.
Scoop a heaping tablespoon of pastry cream onto each bottom piece, add a top piece, drizzle with chocolate ganache, and serve.