Gently warm raspberries with the icing sugar until jam-like.
Make up brownie mix as per instructions and pour into oblong tin.
Randomly top with the marshmallow fluff and the raspberries.
Swirl slightly with a skewer.
Scatter over the pistachio nuts.
Bake for approx 25-30 mins until just set.
(optional) serve with a dusting of cocoa powder