To make the base, process the graham crackers to make rough crumbs and then add the butter and cocoa. Process again until it makes damp, clumping crumbs and then tip them into the pan. Press the crumbs into the bottom of the pan to make an even base and put into the freezer while you make the filling.
Put a kettle on to boil.
Melt the chocolate either in a microwave or double boiler, and set aside to cool slightly.
Beat the cream cheese to soften it, then add the sugar and custard power, beating again to combine. Beat in the whole eggs and then the yolks, and the sour cream. Finally add the cocoa dissolved in hot water and melted chocolate and mix to a smooth batter.
Take the springform tin out of the freezer and line the outside of the tin with a good layer of cling wrap, and then another layer of strong foil over that. This will protect it from the water bath.
Sit the springform tin in a roasting pan and pour in the cheesecake filling. Fill the roasting pan with just boiled water to come about half way up the cake tin and bake in the oven for 45 minutes to 1 hour. The top of the cheesecake should be set, but the underneath should still have a wobble to it.
Peel away the foil and cling film wrapping and sit the cheesecake in its tin on a rack to cool. Put in the refrigerator once it is no longer hot, and leave to set, covered with plastic overnight. Let it lose its chill before unspringing the cheesecake to serve.
To make the chocolate sauce: very gently melt the chopped chocolate, cream and syrup. When the chocolate has nearly melted, take off the heat and whisk it to a smooth sauce. Let it cool a little, and pour it over the chocolate cheesecake on its serving plate.