Preheat oven to 350°F. Grease an 8-inch square baking pan.
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 a low speed until combined.
Divide dough in half and press half of dough into an even layer on
the bottom of the prepared pan. Place chocolate bars over dough. 2
king-sized Hershey’s bars should fit perfectly side by side, but
break the chocolate (if necessary) to get it to fit in a single layer no
more than 1/4 inch thick. Spread chocolate with marshmallow
creme or fluff. Place remaining dough in a single layer on top of the
fluff (most easily achieved by flattening the dough into small
shingles and laying them together).
Bake for 30 to 35 minutes, until lightly browned. Cool completely
before cutting into bars.