Line a slice tin with enough baking paper that it is hanging over the sides
In a large bowl, crush the Marie Biscuits until the resemble fine crumbs.
Melt the chocolate and butter together. I use the microwave to melt chocolate, cooking it for 30 second spurts on a medium high heat and mixing in between.
Add the melted chocolate and butter and the condensed milk to the crushed biscuits. Using a large metal spoon, stir until combined.
Pour mixture into the slice tin, and use the back of your metal spoon to press it down into the tray, creating a flat surface.
Melt the white chocolate in the microwave, and drizzle across the top of the slice.
Place slice in the fridge to firm, and after approximately 1 hour, remove and cut into small slices.