Preheat your oven to 177 degrees Celcius and line an 8x8 inch square baking pan.
Using a handheld mixer in a large bowl, cream together the butter and the sugar together for 2 minutes. Add the eggs and vanilla extract and mixed until combined (if it looks curdled don't worry).
Add the flour, baking powder and salt together and mix them together.
Pour the flour mix into the wet ingreadients and mix together until combined.
Press a small layer of the dough mixture into the prepared tin then add the marshmallow cream on top.
Add the chocolate chips to the remainder of the cookie mixture and press that on the top of the marshmallow cream.
Bake for 25-30 minuites until golden brown.
Remove from the oven and leave the bars to cool completley and then cut them in to 12 squares.
