Add together softened butter sugar and granulated sugar mix together and then add one egg and one egg yolk and vanilla and whisk these in add plain flour mix until combined then add your chocolate chips and fold these in and scoop half the mix into a baking sheet and spreading out evenly then add a layer of chocolate followed by a thick layer of marshmallow fluff spread out evenly then top with the rest of the blondie mix and spreading it out to the edges then bake 350 Degrees For-25mins