Heat a large skillet over medium heat and add the butter. Once it melts, add the peach slices and flip once to cover them in butter. Cook for 3 minutes, until the peaches begin to soften, then sprinkle on the brown sugar. Flip the peaches once more so both sides can be covered in sugar. Cook for another 5 to 6 minutes, flipping once or twice more until the peaches are golden and caramely.
Butter the outsides of the bread slices. Spread the mascarpone on the inside of the bread slices. Assemble the sandwiches by adding the peaches o top of the mascarpone and then the chopped chocolate. Cover with another slice of bread and add to the same skillet over medium heat. Cook until the bread is golden and the mascarpone is melting. Very gently flip the sandwich so the peaches don’t fall out.
note: you can add a little brie cheese on top of the mascarpone if you’d like! I’ve tried it and it’s RIDIC.