Heat oven to 180C/160C fan/gas 4 and lightly oil a non-stick loose-based 22cm cake tin. Melt the butter in a pan and stir in the oatcakes, crackers, cocoa and the chopped peanuts until well coated. Press into the base of the tin to make a firm layer, then bake for 10 mins. Remove from the oven and turn up the heat to 240C/220C fan/gas 9.
To make the topping, mash the bananas with the lemon juice in a large bowl, then beat in the ricotta, yogurt, eggs, vanilla and cornflour until everything is very well mixed. Pour onto the biscuit base and bake for 10 mins, then turn the oven right down to 110C/90C fan/gas 1/4 and cook for 25 mins more. Turn off the oven and leave the cheesecake inside to cool. When cold, chill for 3-4 hrs. (Cooling the cheesecake this way should prevent it from cracking.)
To serve, remove the cheesecake from the tin and slide onto a plate. Whip the cream until it just holds its shape, then smooth over the top of the cheesecake, lightly dust with cocoa and scatter over the peanuts.