Grease and line a 8" (20cm) cake tin
Melt butter over a low heat in a large pot.
Add chopped chocolate, and stir until fully melted and smooth.
Whisk in cocoa, sugar, egg, vanilla and raisins. Stir until well mixed.
Add crushed cookies, stir until all are well coated.
Press firmly into your cake tin.
Make your topping. Melt chopped chocolate in your microwave, or in a bowl over a pot over simmering water. Add coconut oil, stir until blended, then pour over your slice. Spread evenly with the back of a spoon. Leave as is, or top with your favourite topping, then pop into the fridge.