Preheat oven to 180 degrees C. Grease a 20x30cm pan.
In a medium bowl, mix together the melted butter and 3 1/2 cups crumbs. Press half of the mixture into the bottom of the prepared pan. In the microwave or in a metal bowl over a pan of simmering water, melt chocolate chips, stirring frequently until smooth. Remove from heat and spread over the crumb layer. For easier spreading, refrigerate the crust while the chocolate is melting. Make a layer of marshmallows over the chocolate and top with the remaining crumbs.
Bake for 15 minutes in the preheated oven, cool and cut into squares.