Cut bread diagonally into 1 1/2 inch slices to within 1/2 inch of bottom. Repeat cuts in opposite direction. Cut cheese into 1-inch squares. Place one cheese and one mushroom if desired into each slit. Combine butter and garlic powder; spoon over the bread. Place on an ungreased baking sheet. Bake at 350° for 8-10 minutes or until cheese is melted.