Beat the butter until it is fluffy in a large bowl. Add 1 cup of Parmesan cheese, sour cream, Italian seasoning, and dry mustard. Beat again until fluffy. Gradually add the flour and blend with wooden spoon. The dough will be stiff. Cover and set the dough in the refrigerator for 40 minutes. Roll the dough out on a floured surface. Mix the egg yolk with the water. Brush mixture over the rolled out dough with a pastry brush. Sprinkle the remaining ¼ cup Parmesan cheese over the dough. Cut the dough into strips, twist, and place on a greased cookie sheet. Bake at 350 degrees for 20-25 minutes, or until golden brown. Store twists in a tightly-sealed container once cool.