Line a 20cm x 20cm square cake tin (which has high sides) with baking paper. Make sure you leave plenty of baking paper hanging over the edges to help you remove the Tim Tam Hedgehog Slice once it has set.
Roughly chop SIX of the Tim Tams into large pieces and set aside.
Place the plain biscuits into a food processor and blitz until they have just broken up, then add the remaining whole Tim Tams (approximately 1 ½ packs) and blitz for 5 seconds or until they have broken into large chunks - alternatively you can simply crush the plain biscuits with a rolling pin and roughly chop the Tim Tams with a large knife.
Place the crushed biscuits into a large bowl and set aside.
Pour the condensed milk into a small saucepan and add the butter and cocoa. Cook over a low heat stirring occasionally until the butter has melted and combined with the other ingredients.
Pour the melted ingredients into the bowl with the crushed biscuits and add the coconut. Working quickly, use a large metal spoon to mix the ingredients together.
Pour the mixture into your prepared tin and flatten the surface with the back of a spoon.
Break the milk chocolate into squares and place it into a microwave safe bowl. Cook in the microwave on high for 30 second spurts (mixing in between) until it's just melted. Pour over the top of the slice and sprinkle the Tim Tams you set aside earlier over the top. Place the Tim Tam Hedgehog Slice into the fridge for 6 hours (preferably overnight) to set before cutting into squares.
You can use a rolling pin to crush the biscuits instead of a food processor if you prefer.
You can also make this slice in a 18 x 28cm slice tin.
Some of the Tim Tams will melt when you add the melted butter mixture - this is ok!