In another large bowl, add 5 cups of marshmallows, 1½ cups of chocolate, light corn syrup and butter. Microwave in 30-second increments, stirring after each one until the mixture is totally melted and smooth. Stir in vanilla.
Pour the delicious melted mixture over the grahams, put a little cooking spray on a spatula and gently stir until all of the grahams are coated. Stir in the extra cup of marshmallows.
Turn the mixture out into the prepped baking pan. Use your spatula covered with cooking spray, and press the mixture evenly into the pan, pushing it down into a flat, even layer. Let set at room temperature for at least 1 hour or pop them into the fridge to firm up faster. Store in an airtight container at room temp for up to 2 days.