Grease with a little butter and line a square cake pan of approximately 8.60 inches (22 cm) wide with parchment paper.
Place the butter in a large saucepan and bring to medium heat so it melts. Add 2/3 parts of the marshmallows with the vanilla extract and cook for a few minutes, using a rubber spatula to stir the marshmallows until completely melted, scraping down the sides to ensure the marshmallows do not burn. Then remove the pan from the heat immediately.
Quickly add the rice krispies cereal with the golden grahams cereal and fold into the mixture with the same rubber spatula. Add the rest of the marshmallows along with the chocolate chips and mix until the ingredients are perfectly combined.
Pour the mixture into the cake pan and carefully press with the rubber spatula or your hands to form an even layer. It is important not to press too hard or the bars turn very compact.
Decorate the bars with half of the chopped chocolate and press a little so that they stick together well. The chocolate may start to melt, so reserve half of the amount for once the bars have cooled down.
Let the bars cool down for 1 hour. Turn them out of the pan and decorate with the remaining chopped chocolate and cut into 16 equal squares.