i personally like to line the pan with foil before i put the batter in so after your done you can easily take all the brownies out of the pan for easy cutting. also spray your knife with Pam so the marshmallow doesn't stick

make brownies according to the box. pour batter into 13x9 pan

place reeses cups as desired in the pan. Bake at 350 for 24 minutes

once the brownies have baked let them cool. microwave jet puff marshmallow in 10-15 sec increments and stir until spreadable. spread on top of brownies and also allow marshmallow to cool.