In a bowl, mix the condensed milk with the cacao powder very well.
Crumble in the biscuit using a mixture of small pieces and crumbs, and combine well, ensuring all the biscuits are coated.
Spread out a large piece of plastic wrap on the counter, and spoon the mixture in the middle.
Mould it into a log shape by wrapping the cling film around it and twisting at each end.
Cover the log with foil and freeze for at least 8 hours or preferably overnight (depending on the power of your freezer, the 8 hours may not be enough).
To serve, unwrap the foil and start peeling the plastic wrap up to the point you want to cut, then cut ½ inch to 1 inch thick slices with a sharp knife that has been dipped in hot water. Serve immediately
If you struggle to create a log shape, you can line a cake pan with cling film and spoon in the mixture inside for square/rectangular shapes.
Don't skip the plastic wrap! If you only use foil it will stick to the cake once frozen. The plastic wrap however will peel right off before you cut it.
If the plastic wrap starts sticking to it, it means you need to freeze it for longer. Pop it back in the freezer and try again. Optimum freezing time is overnight.
The lazy cake can stay in the freezer for a few months. Just ensure that you return it to the freezer after you're done cutting