Spread some pesto on the inside of the bread.
Grill the pesto side of the bread for about 2 min on medium heat. It should come off the grill easy with a spatula when it's cooked long enough. Be careful not to burn it.
Spread butter on one side and starting grilling on that side first. Spread butter on other side when you start grilling.
Grill for about a min on each side for over medium fire.