Lay out bread and spread pesto on both sides.
Slice tomatoes and mozzarella cheese into thin slices.
Place tomatoes and mozzarella on slices of bread.
Put sandwiches together and spread softened butter on the outside of the top piece of bread.
Place buttered bread side down on a skillet that is already warmed up on medium heat.
Butter the other side of the sandwich.
Flip when one side is golden brown, about 2 minutes.
Place a large lid from another pan to cover sandwich while cooking.
Cut and serve.