Spread the peanut butter on 2 slices of the bread, dividing evenly. Top with the chocolate and sandwich with the remaining 2 slices of bread.
Heat the butter in a large nonstick skillet over medium-low heat. Add the sandwiches and cook, covered, until the chocolate is melted and the bread is golden brown, 4 to 5 minutes per side.
Slice each sandwich into quarters.