To make the base, process the graham cracker crumbs, butter and cocoa powder in a food processor until it makes damp, clumping crumbs. Tip them into a 9-inch springform 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.
Preheat the oven to 350 degrees F. Put a kettle on to boil. Melt the chocolate either in a microwave or double boiler, and set aside to cool slightly.
To make the filling, beat the cream cheese to soften it, and then add the sugar and custard powder, 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 the melted chocolate and mix to make a smooth batter.
Take the springform pan out of the freezer and line the outside of the pan with a good layer of plastic wrap, and then another layer of strong foil over that. This will protect it from the water bath. Sit the springform pan in a roasting pan and pour the cheesecake filling into the springform pan. Fill the roasting pan with the just-boiled water from the kettle to come about halfway up the cake pan, and bake in the oven 50 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 plastic wrap and sit the cheesecake in its pan on a rack to cool. Put in the fridge 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 glaze, very gently melt the chopped chocolate, cream and corn syrup. When the chocolate has nearly melted, take it off the heat and whisk until smooth. Let it cool a little, then Jackson Pollock it over the cheesecake. Use the remaining glaze as an accompanying sauce.