Prepare six Texas Toast slices according to package directions. Shape beef into 12 thin patties. Top six patties with 1 T. of pizza sauce, 3 slices of pepperoni and 1 slice mozzarella cheese. Top with remaining patties, pressing edges firmly to seal. Grill covered, over medium heat for 8 minutes on each side or until done. Meanwhile, in a skillet, saute mushrooms in butter. Top each burger with mushrooms and Parmesan cheese and serve on Texas Toast.