Preheat the oven to 350°F/180°C.
In a large bowl, cream butter and sugar until well blended. Add eggs and vanilla; beat well.
In a separate bowl, stir together flour, graham cracker crumbs, baking powder and salt; add to butter mixture, beating until well blended.
Press half of dough in a prepared 9x13-in. pan. Reserve remaining dough. Bake 15 minutes.
After removing from oven, arrange mini or large chocolate bars over baked layer, breaking as needed to fit. Sprinkle with marshmallows; scatter bits of remaining dough over marshmallows, forming top layer.
Bake 10 - 15 more minutes or until lightly browned.
Cool completely in pan on wire rack. Cut into bars.
Recipe is included with our Kitchen Essentials from Calphalon 9x13-in. Covered Cake Pan.