Grease and line a 9"x9" slice tin with baking paper and set aside.
Place the butter, cocoa, vanilla extract and dark chocolate in a saucepan and melt together. Or melt in a microwave-safe bowl.
Crush the digestive biscuits in a large bowl and pour over the melted mixture. Stir until well combined
Spread the mixture evenly into the prepared tin, pressing down firmly and place in the fridge for around an hour while you make the icing.
Place the icing sugar, cocoa and softened butter in a bowl.
Pour in two tablespoons of water to the bowl and mix ensuring the water melts the butter through the mixture. If the mixture is too stiff add more water one teaspoon at a time until you have a thick but smooth consistency.
Spread over the icing in an even layer and return the slice to the fridge for at least another hour before slicing and serving.
Store in an airtight container in the fridge for up to two weeks.