Step 1

Position the racks to divide the oven into thirds and preheat it to 425°F. Line two baking sheets with parchment paper.

Step 2

Bring the milk, butter, and salt to a boil in a medium saucepan over high heat. Add the flour all at once, lower the heat to medium and, using a wooden spoon, immediately start stirring energetically. The dough will form into a ball and there will be a light film on the bottom of the pan. Keep stirring for another two minutes or so to dry the dough: Dry dough will make puffier puffs.

Step 3

Turn the dough out into the bowl of a stand mixer fitted with the paddle attachment (or work by hand in a large bowl with a wooden spoon and elbow grease). Let the dough sit for a minute to cool, then add the eggs one by one, followed by the white, beating on medium speed until each one is incorporated before adding the next. The dough may look as though it's separating or falling apart -- just keep going, and by the time the egg white goes in, the dough will be smooth. Beat in the mustard, followed by the cheese. Give the dough a last mix-through by hand.

Step 4

Scoop or spoon out heaping tablespoon-sized balls of dough (I use a 1.5-tablespoon cookie scoop), and drop the dough on the prepared baking sheets, leaving about 2 inches between the mounds. (The scooped dough can be frozen on the baking sheets.)

Step 5

Slide the baking sheets into the oven and immediately turn the temperature down to 375°F. Bake for 15 minutes, then rotate the pans from front to back and top to bottom. Continue baking until the gougères are puffed, golden, and firm enough to pick up, another 15 to 17 minutes. Serve immediately -- these are best directly from the oven. Storing: Although the puffs are best served hot out of the oven, they are still nice (although flatter) at room temperature that same day. If you want to keep baked puffs longer, freeze them and then reheat in a 350°F-oven for a few minutes.

Step 6

Make-Ahead/Freezer Instructions: Scoop the dough and freeze on a parchment-lined baking sheet, then pack them in an airtight container. You can bake them straight from the freezer; just give them a few more minutes in the oven.