Preheat the oven to 450°F, and line a baking sheet pan with a silicone mat (parchment paper can burn at this high of temperature!).

In a 2-quart saucepan with a heavy bottom (enamel-coated cast iron is best), add the butter, warm water, salt, and mustard powder (if using). Turn the heat to medium-high and melt the butter into the water.

When the butter and water mixture is at a rolling boil, dump in all of the flour at once.

Stir the mixture immediately, and then lower the heat to medium-low.

Continue to cook and stir the mixture for about 4 minutes, or until the dough is stiff, appears dry, and is clumping together in a ball around the spoon.

Move the dough mixture to a mixing bowl, and beat with an electric mixer for 1-2 minutes, until it is no longer hot to the touch.

While continuously beating, add 1 egg. Once the egg is fully incorporated, add the other egg while still beating.

Once the mixture is homogenous, it will be a soft, scoop-able dough.

Finally, beat in the grated cheese and chopped chives.

Using a cookie scoop that holds about 1 tablespoon of batter, scoop out 1 dozen balls and space them evenly on the baking sheet.

Bake for 5 minutes at 450°F, and then lower the heat to 350°F, and continue baking for another 20-25 minutes, until the puffs are golden brown.