Butter each slice of bread and flip them over onto a large, flat surface. Spread the mayonnaise over 2 slices of bread and layer with tomato, cheese, and pepperoni. Assemble the sandwiches, buttered side facing up.
Grill it:
Heat a non-stick pan over medium-high heat. Grill the sandwiches, 1 at a time, for 2 to 3 minutes or until the cheese begins to melt. Flip and grill for an additional minute until the cheese melts.
Remove the sandwich from the pan and allow it to cool for 5 minutes.
For a packed lunch:
Once cooled, pack the sandwich into a lunch container, separated from other ingredients like fruit and veggies.