Heat your oven to 350 degrees and prepare the brownie batter according to the directions on the box. Grease 12 small bowls (that can hold 3/4 cup of liquid) with butter. Pour 2 heaping tablespoons of the brownie batter into the bottom of the bowls, and place them on a baking sheet and then into the oven to bake for 15 minutes.

After 15 minutes, remove the bowls from the oven, and add a marshmallow on top of the brownie batter in each bowl. The brownie will not be fully cooked - and that is ok. Return the bowls to the oven for a minute or two, until the marshmallow has melted. Then break up the chocolate bar into squares, and place a square on top of the marshmallows. Lastly, flatten out the ready to bake chocolate chip cookies and place them on top of the chocolate bar. Return to the oven for about another 15 minutes - until the cookie on top has browned.

Let cool for 5 minutes and then serve - this is best when warm.