1Heat evaporated milk just until boiling. Add chocolate, let stand for 1 minute, then whisk until smooth. Let cool slightly.
2Spread 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 and nuts, if using, into the chocolate. Cover with the remaining slices of bread, buttered-side up, and press lightly.
3Cook the sandwiches in a large nonstick skillet over medium-high heat for 1 to 2 minutes. Turn over, press with a spatula, and cook until nicely browned and the chocolate is barely melted, 30 seconds to 1 minute. Serve warm.