Grease and line a 7inch square baking tin. Break up your milk chocolate into a bowl and pop in the butter. Melt together in the microwave for 30 seconds and then stir. Continue to heat in 10 second bursts, stirring between bursts until your mixture has melted. Set aside to cool briefly.
Stir in the cookies, mini marshmallows and sultanas together in a bowl. Pour over the melted chocolate mixture and stir until it is all covered in chocolate. Transfer the mixture into the prepared tin, press down and allow to cool. Chill for at least 2 hours until set, then slice and serve.