Heat a Panini grill. Place the four slices of bread on a cutting board and brush lightly with the melted butter, being sure to butter the corners. Turn the slices over and pile Gruyere on two of the slices. Place the remaining two slices of bread on top of the Gruyere, buttered sides up.

Grill the sandwiches on the Panini grill for about 5 minutes, until nicely browned. Place on a cutting board, Allow to rest for 1 minute and cut into 1–inch cubes.