Line a baking tray with non-stick paper, and lightly grease.
Cut the cookie dough into chunks, and press it onto the paper to form a rough circle. Bake according to the instructions on the packet - mine took around 10-15 minutes at 190°C (Gas Mark 5 / 375°F).
When the cookie is just cooked, set aside.
When it has completely cooled, spread it with marshmallow fluff.
Roughly chop the Oreos, milk chocolate and white chocolate, and sprinkle over the cookie.
Cut into slices to serve.