1. Preheat the oven to 175°C. Grease an 8-by-8-inch baking pan and line with aluminum foil.
2. Cream together the butter and sugars in a large mixing bowl until light and fluffy. Beat in the vanilla and eggs until fully incorporated.
3. Mix together the flour, biscuit crumbs, baking powder and salt in a separate mixing bowl. Slowly add the dry ingredients to the wet ingredients until combined. Divide the dough in half.
4. Press half of the dough into the bottom of the prepared pan. Sprinkle the chocolate evenly over the dough. Top the chocolate with the marshmallow creme.
5. Spread the marshmallow creme in an even layer, using a rubber spatula that has been sprayed with nonstick spray.
6. Roll out the remaining dough to an 8-by-8-inch square on wax or parchment paper, using a rolling pin.
7. Working carefully, place the dough on top of the marshmallow layer Bake until the crust is golden brown, 30 minutes.
8. Completely cool before cutting into bars.