Preheat the oven to 200˚C/Gas Mark 7.

Rub some butter around the inside of a 20x15cm/8x6” baking tray or insert a baking liner.

Empty the brownie mix into a large mixing bowl. Add the milk and stir into a smooth batter. Add the melted butter and beat well until combined.

Tip the mixture into the prepared tray, spreading it out to the edges, scatter over the marshmallows, then bake in the oven for 14-17 minutes.

When cold cut into squares or rectangles before serving