¾ cup flour
½ teaspoon sugar
½ teaspoon salt
1/8 teaspoon nutmeg
pinch cayenne pepper
3 ½ tablespoons unsalted butter
¼ cup whole milk
3 large eggs
5 ounces Gruyère, grated
1 egg, beaten with 1 teaspoon water.
Mix the flour, sugar, salt, nutmeg and cayenne in a small bowl. In a heavy saucepan, melt the butter with the milk and ¼ cup of water over medium heat, stirring constantly with a wooden spoon. Bring to a rolling boil, add the flour mixture and stir briskly for 1 minute. The dough should form into a ball, and a thin film should cover the bottom of the pan.
Immediately transfer the dough to the bowl of an electric mixer fitted with a paddle. Mix on low to quickly release the steam. Just after the steam subsides, add an egg and increase the speed to medium. The dough will break into lumps at first. Once the dough comes back together, add the second egg and continue mixing.
In a small bowl, lightly beat the third egg. Stop the mixer. When the dough is lifted with a spoon, it should detach and form a slowly bending peak. If the dough is too thick and doesn’t bend, mix in half of the beaten egg. Check the dough again; add the remaining beaten egg as needed.
Preheat the oven to 400 degrees. Line two sheet pans with parchment paper.
Fold ¾ of the Gruyère into the warm dough. Pipe or scoop the dough onto the prepared pans in tablespoon-size balls. Using a pastry brush, glaze each ball with the egg wash, smoothing out peaks with gentle pressure on the brush. Top each ball with a pinch of the remaining grated cheese. Bake for 18 minutes, rotating the pans once. The puffs should be light golden brown. Turn off the oven, crack open the oven door and let cool in the oven for at least 10 minutes before serving. Serve immediately.