Prepare your station: Gather all your ingredients and place them near your cooking space.
Butter the bread: Spread a thin layer of butter on one side of each slice of bread. This will make our grilled cheese delightfully crispy.
Assemble the sandwich: Place one slice of bread, butter-side down, onto a skillet over medium heat. Add half of the shredded mozzarella cheese, followed by the marinara sauce, pepperoni slices, and the remaining mozzarella cheese. Top with the second slice of bread, butter-side up.
Grill it: Cook for 4-5 minutes until the bottom is golden brown. Carefully flip the sandwich and grill the other side for another 4-5 minutes until golden brown and the cheese is melted.
Slice and serve: Remove the sandwich from the skillet, slice it in half, and serve with extra marinara sauce for dipping. Garnish with fresh basil or a sprinkle of Parmesan!