Line a brownie tin with baking paper and set aside.

Fill a snap-lock bag with the arrowroot biscuits, squeeze out excess air, and hit with a rolling pine on a hard surface until the biscuits are crushed.

In a large bowl, combine the crushed biscuits, desiccated coconuts, pistachios and cocoa and set aside.

In a small saucepan over a low heat, combine the dark chocolate, butter and condensed milk together for 4-5 minutes, stirring continuously. Remove from heat when mixture is completed melted and combined, and allow to cool slightly. Stir into the dry ingredients to ensure completely combined.

Press the mixture firmly into the prepared pan, and refrigerate for 30 minutes or until firm.

Just before the end of the 30 minutes, place the white chocolate in a microwave safe bowl, and microwave uncovered in 30 second increments, until completely melted. Add the oil and stir to combine, and then spread the mixture over the slice. Refrigerate for an additional 30 minutes, or until set.

Use a hot knife to cut the slice into squares, and keep in the fridge until serving.