Brownies could be made in a cast iron skillet to good effect. I would suggest a caramel sauce and slicing into wedges.
If you really want a sandwich, for whatever reason, I would suggest pan-toasting two pieces of bread in butter, flipping, putting some bittersweet chocolate on one piece of bread…. cover, allow the chocolate to melt. Pull the bread out of the pan, smear cream cheese on the other piece, smoosh together (cheese to chocolate).
This will work better with a dense crumb white bread, like "italian" as opposed to Wonder bread or any of the similar squishy big brands… unless they are well toasted to stiffen the crumb. Whole wheat breads would be problematic, I expect.
Honestly, I think you would be happier walking down to the corner store, buying a chocolate hand pie and microwaving it for a few seconds.
If you wanted to try and improvise a hand pie out of bread, that might have potential. Not sure if it would be better to use two pieces or one piece folded on the diagonal. Either way, that is a definite job for Wonder bread. Take a Milky Way bar, chop up about half of it (1/4 for a single slice), sprinkle on the bread. Either top with another piece or fold in half. Trim crusts, press edges together with a fork to make a pocket sandwich. Toast in the frying pan. Flip it often.