Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13 inch pan.
In a medium bowl, mix together the melted butter and graham cracker crumbs. Press half of the graham cracker 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 graham cracker 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 graham cracker crumbs.
Bake for 15 minutes in the preheated oven, cool and cut into squares.