Blend 2 packets of biscuits in a blender, along with the softened butter. Cover a small rectangular tray with plastic wrap.
Pour the mixture into it, press and smooth out with a spoon.
Blend the other 2 packets of biscuits with the chocolate. Add the strained yoghurt, cereal and raisins and stir with a spoon.
Pour it over the biscuit layer. Smear with chocolate spread on top, put away in the fridge for 4-5 hours.