Crush biscuits in a zip lock bag (double bag) with a rolling pin until the biscuit reaches a mixture of bigger chunks and finer crumbs.
Place butter, cocoa powder, sugar and vanilla into a pot. Melt over medium heat, stirring consistently, until the sugar has completely dissolved.
Remove from the heat and add in the crushed biscuits, stirring to combine.
Line a square slice tin with baking paper.
Press the mixture into the slice tin, making sure the top is level. Place into the refrigerator for 30 minutes to firm up and cool before icing.
For the icing: sift icing sugar and cocoa powder in a bowl. Mix in the softened butter and milk. Beat together until light and fluffy.
Spread the icing over the slice and sprinkle over the desiccated coconut (if using).
Place back into the refrigerator until chilled. Remove and cut into slices. It will keep for up to week stored in an airtight container. Enjoy!