Spread butter on one side of each slice of bread. Divide the chocolate mixture on the unbuttered side of 4 slices, leaving a little border. Press chocolate chips into the chocolate. Cover with the remaining slices of bread, buttered-side up, and press lightly.
Cook the sandwiches in a large nonstick skillet over medium-high heat for 1 to 2 minutes. Turn over, and cook until nicely browned and the chocolate is barely melted. Serve warm.