Lay out the 4 pieces of brioche bread. Lightly press in the center of 2 of the pieces to make a divot. Fill the divot with the marshmallow chocolate ganache.
Moisten the edge of the bread by dipping your finger in water and running it along the edge. Set the other pieces of bread on top and lightly press the edges to seal.
In a shallow bowl combine the milk, egg, vanilla, cinnamon, and salt. Whisk until fully combined.
Place the crushed graham crackers on a shallow plate.
In a large saucepan heat the butter over medium-high heat.
Carefully dip the stuffed bread into the milk mixture and transfer it to the plate with the crushed graham crackers. Cover the stuffed bread with the crushed graham crackers.
When the pan is hot place the stuffed bread in the pan and cook for about 2 minutes on each side, or until golden brown.
Top the french toast with any left over marshmallow chocolate ganache and serve warm.