Begin by assembling your sandwiches. Spread peanut butter on two slices of bread. On top of the peanut butter, layer the chocolate and marshmallows. Place the remaining slices of bread on top to create a sandwich.

In a shallow bowl, whisk together the egg, milk, sugar, and cinnamon until well combined. This will serve as your custard mixture.

Heat a skillet over medium heat and add one tablespoon of butter, allowing it to melt and coat the pan evenly.

Dip each sandwich into the custard mixture, ensuring both sides are well coated. Allow any excess mixture to drip off before placing the sandwich in the skillet.

Fry the sandwich for about 2-3 minutes on each side or until golden brown and crispy. Add more butter to the skillet if needed for frying the second sandwich.

Once cooked, remove the sandwiches from the skillet and place them on a paper towel to absorb any excess grease.

Slice the sandwiches in half to reveal the ooey-gooey filling. Dust with powdered sugar if desired, and serve warm for the best experience.