Spread desired toppings evenly on one side of 4 slices of bread; set aside. Heat butter in an extra-large nonstick skillet over medium-high heat until it bubbles. Loosely pile individual slices of mortadella in pan; cook and turn occasionally until mortadella is well browned and crispy on the edges (about 5 min.). Pile fried mortadella and cheese evenly on top of prepared bread; place remaining slices of bread on top. Cook sandwiches in the same pan used to cook the mortadella, over medium-high heat, adding a bit more butter, if desired. Cook on first side until the bottom is dark golden-brown (about 2 min.); flip and repeat. If needed, reduce heat slightly and continue cooking, flipping occasionally, until sandwich is hot and bread is dark golden-brown. Cut each sandwich in half on the diagonal; serve immediately.