Preheat the oven to gas mark 4/180ºC/350ºF.To make the base, process granola with the butter until well mixed then tip into a 23cm spring form tin
Press well into the bottom of the tin to make an even base and put into the freezer while you make the filling
Melt the chocolate in a bowl over a pan of simmering water and set aside to cool slightly
Whip the cream cheese with the sugar and custard powder until smooth then beat in the whole eggs and then the yolks, and then stir in the yoghurt
To this mixture add the melted chocolate, and mix to a smooth batter
Take the spring form tin out of the freezer and line the outside of the tin with a good layer of cling film, and then another layer of foil over that
Sit the spring form tin in a roasting pan and pour in the cheesecake filling
Fill the roasting pan with just-boiled water to come about halfway up the cake tin, and bake in the oven for 45 minutes to an hour
The top of the cheesecake should be set with a wobble underneath
Remove the foil and cling film and sit the cheesecake on to a rack to cool
Once cool place in the fridge and leave to set, covered with cling film, overnight
To serve remove from the fridge and allow to sit for 30 minutes or so. Carefully undo the spring form tin, loosening the cake with a thin bladed knife dipped in hot water if necessary