Spread a thin layer of mayonnaise on one side of each of the 8 slices of bread. This will be the outside of the sandwich.

Place 4 slices of bread, mayo-side down, on a cutting board. Layer 1 slice of mozzarella cheese onto each piece of bread.

Arrange 9 slices of pepperoni evenly over the cheese on each sandwich.

Spoon 1 tablespoon of pizza sauce into the center of the pepperoni layer on each sandwich.

Place 1 more slice of mozzarella cheese over the sauce and pepperoni, then top with the remaining bread slices, mayo-side facing UP.

Heat a large skillet over medium-low heat. Place sandwiches in the skillet and grill for 4-5 minutes until the bottom bread is golden brown.

Carefully flip the sandwiches and cook for another 3-4 minutes until the second side is golden brown and the cheese is fully melted.

Remove from heat, let rest for 1 minute, slice diagonally, and serve immediately.