Once the butter has completely melted, turn the heat down to medium-low, add the flour, and stir continuously for about 5 minutes, mashing and stirring.
4 ounces (1 cup) unbleached all-purpose flour
Let the mixture cool for a minute or two and then put it into your food processor (or stand mixer).
In a food processor, remove the feed tube to let the hot air escape. Turn the machine on and let it run for a minute to cool the mixture.
Add the eggs very slowly while the machine runs and then process or mix for about one minute. The mixture will be shiny and still liquid enough that it does not hold firm peaks.
4 eggs, lightly beaten
Add the bacon, cheese, and salt, and process or mix only to combine.
2 slices bacon, cooked, drained, and chopped fine, 1 ounce gruyere or other cheese, grated, ½ teaspoon salt
Transfer the mixture to a piping bag or plastic gallon storage bag with a corner cut out. Pipe the mixture onto the prepared baking sheets, keeping the puffs about an inch apart. If the puffs have peaks after piping, smooth them down with a wet finger.
Put the baking sheets into the oven and turn the oven down to 425°F. Bake for 10 minutes.
Turn the oven down to 375°F and bake for about another 20 minutes until the puffs are firm on the outside and mostly dry on the inside (you'll just have to try one!).
If the outsides are brown but the insides are not completely dry, turn off the oven and leave the baking sheets in the oven with the door slightly ajar for another 30 minutes or so.
Notes
Variation: use a different cheese like Parmesan or blue cheese for a change in flavor.