Pre-heat oven to 350 degrees
Grease 8in. square baking pan
In large bowl beat butter and sugar until fluffy.
Add egg and vanilla
Stir together flour, graham cracker crumbs, baking powder and salt then add to butter mixture beating until mixed.
Press ½ dough into bottom of pan.
Arrange chocolate bars over dough.
Spread marshmallow crème over chocolate.
Scatter bits of remaining dough over top.
Bake for 25 minutes or until lightly brown.
Cool completely then cut.