Heat oven to 350ºF. Grease an 8 inch square baking pan.
Beat butter and sugar together until well blended in a large bowl. Add egg and vanilla; beat well.
In another bowl stir together flour, graham cracker crumbs, baking powder, and salt. Add to butter mixture, beating until blended.
Press half the dough into the prepared pan. Bake 15 minutes.
Unwrap chocolate bars; arrange over baked layer. They may not quite all fit, you may be forced to eat a couple pieces. Sprinkle with marshmallows. Scatter remaining dough over the marshmallows, forming a top layer.
Bake 10- 15 minutes or until just lightly browned. Cool compleletly before cutting.