Grease and line a 20cm square cake tin. Make sure the paper is higher than the sides of the tin for ease of removal.

Chop biscuits in Thermomix bowl by pulsing on speed 4 to 5. You want the biscuit to be still chunky. I break up larger pieces with my fingers. Set aside.

If using nuts place them into the Thermomix bowl and chop again by pulsing on speed 4 to 5. Set aside with biscuits.

Don't wash bowl. Add dark chocolate, broken into pieces and butter. Melt at 50/speed 1 - 2/5 minutes.

Add in milk, powders and sultanas and stir on reverse/speed 2/20 seconds or so. Use spatula to add biscuits or biscuit and nut mix.

Spoon into prepared tin and smooth the top with spatula or wet hands. Refrigerate for an hour or so.

Melt milk chocolate and coconut oil in Thermomix at 50/speed 1 - 2/5 minutes. Meanwhile, melt white chocolate and coconut oil on 50% power in a pyrex bowl until liquid. Remove a tiny bit and mix with powders. Stir back into remaining melted white chocolate.

Pour milk chocolate over slice and smooth out to cover evenly. Place coloured and flavoured white chocolate into a small ziplock bag. Seal, then cut a small hole in one corner. Pipe lines or swirls on milk chocolate topping. Swirl with a skewer if desired.

Refrigerate for two hours.

Remove completely from tin and place on a board to cut into small squares.