Freeze the brioche for at least 30 minutes and then transfer 2 slices to a cutting board.
Evenly coat one slice with 1 ½ tablespoons peanut butter, and the other with 1 ½ tablespoons raspberry preserves.
Sprinkle 1 tablespoon peanuts over the peanut butter-topped slice, and half of the fresh raspberries over the raspberry preserves-topped slice.
Top the raspberries with half of the mini marshmallows.
Flip the peanut butter-topped slice over the marshmallows to create a sandwich.
Preheat a panini press according to the manufacturer's instructions.
Place the sandwich into the preheated press and cook until the bread is browned and the marshmallows are melted.
Transfer the sandwich to a cutting board and drizzle with ½ tablespoon peanut butter. Sprinkle with flaky sea salt and dust with confectioners' sugar, if desired.
Repeat the process with the remaining ingredients.