1Line a 13x9 baking pan with parchment paper. Be sure to leave paper over the edges of the pan for easy cookie bar removal.
In the bowl of a stand mixer with the flat attachment, beat together the butter, sugars, egg and peanut butter until well blended.
In a mixing bowl, beat together the flour, baking soda, salt and baking powder. Add the flour mixture to the butter/sugar mixture and beat until well blended. Fold in the chocolate chips and then the marshmallows.
Press the cookie dough into the 13x9 pan.
Bake at 350 for 15-20 minutes or until the edges are just turning brown.
Cool completely. After cooking, lift the bars from the pan and cut into squares.