Place the sweetened condensed milk, butter and cocoa powder in a small saucepan, and heat over a low heat – stirring until the butter has melted and the mixture is combined. Remove from the heat.
Meanwhile, combine the Marie biscuits, half of the chopped Tim Tams and coconut in a large bowl. Add the butter mixture and stir until well combined. Press into the prepared slice tin.
Melt the white chocolate, and pour over the slice. Sprinkle with the remaining chopped Tim Tams and place into the fridge for a few hours to set. Slice and serve.