Line a slice tray with baking paper
Break biscuits into small pieces (I like to do this by hand, I can control the size of the pieces)
Add biscuit crumbs to biscuits. (I like to add the extra biscuit crumbs to soak up the chocolate mixture)
Melt butter and add cocoa, stir in so the cocoa melts smoothly
Add the condensed milk to the butter mixture and stir until smooth and incorporated well
Combine the biscuits with the condensed milk mixture, stir until biscuits are coated
Press into the prepared slice tray then smooth with the back of a spoon.
Melt the chocolate (I do this in the microwave 1 minute on high, stir then in 30 second bursts till melted)
Add the oil and stir until smooth then pour over slice and sprinkle with sprinkles or 100's/1000's
Refrigerate until set, remove from fridge 30 minutes before cutting into desired size.