In a large bowl, cream together butter and sugar until light. Beat in egg and vanilla.
In a small bowl, whisk together flour, graham cracker crumbs, baking powder and salt. Add to butter mixture and mix at low speed until combined.
Divide dough in half and press half of dough into an even layer on the bottom of a greased 8 inch square baking pan.
Place chocolate bars over dough, breaking the chocolate bars (if necessary) to get it to fit in a single layer no more than ¼ inch thick.
Spread chocolate with marshmallow creme or fluff.
Place remaining dough in a single layer on top of the fluff (easiest if you flatten the dough into small shingles and then lay them together).
Bake for 30 to 35 minutes, until lightly browned. Cool completely before cutting into bars.
Notes
These are best when heated, because everything inside is warm and gooey just like when you make them over the campfire. If desired, after you plate the bars you can microwave them for about 10-15 seconds just before eating.