Step 1In the bowl of a food processor, mix the flour, Parmesan cheese, salt, pepper and dry mustard. Add the butter and work it in using the pulse button so the mixture forms crumbs. Press a few crumbs together with your fingers. If it's sticky, add 2 to 3 tablespoons more flour.
Step 2Butter a baking sheet. Turn the crumbs into a bowl, press them into balls 1 inch in diameter and place them on the baking sheet. Chill in the refrigerator for 30 minutes.
Step 3Heat the oven to 350 degrees. Bake the cheese balls until lightly browned, 26 to 28 minutes. They keep well in an airtight container, or they can be frozen.