Rub together the flour and the butter in a bowl using your fingertips; then work in the egg yolk, Parmesan cheese, salt and mustard powder. Mix to bring the dough together into a ball. Shape the mixture into a log, wrap in foil or plastic wrap and chill for 10 minutes.
Preheat the oven to 400°F. Cut the Parmesan log into very thin slices (1/4 inch thick maximum), and arrange on a baking sheet. Flatten with a fork to give them a ridged pattern. Bake for 10 minutes, or until they are crisp but not changing color.