Heat the oven to 400°F/200°C and lightly butter a baking sheet. Make the pâte à choux. Cut the Gruyère cheese in tiny dice, or coarsely grate it, and beat it into the dough. Transfer the dough to a pastry bag fitted with a ½-inch/1.25-cm plain tip and pipe
Bake the puffs until golden brown and crisp, 25 to 30 minutes. The puffs often seem done too soon, so take out a puff and let it cool for a minute or two to check if it is ready before removing the rest. The puff should stay crisp on the outside and slightly soft inside. Gougères are at their best warm from the oven, but they can be baked ahead, or stored in the freezer, and reheated in a low oven.