Preheat the oven to 180 C / Gas 4 – Grease a 23cm (9 in) springform tin with butter, then wrap the outside of the tin in foil.
Combine the crushed biscuits, 50g butter, 2 tablespoons sugar and cocoa in a small bowl; mix; press into the bottom of the prepared tin.
Place the cream cheese in a large bowl and beat with an electric mixer on low speed until smooth. While beating, slowly add 200g sugar and then the eggs, one at a time. Continue beating until smooth.
Combine the 225g chocolate and double cream in a bowl. Heat in the microwave until the chocolate is melted. Beat the chocolate into the cream cheese mixture. Add the soured cream, salt, Guinness and vanilla; blend until smooth.
Pour the mixture over the base. Place the tin into a large, deep baking dish. Fill the dish with hot water to cover the bottom half of the springform tin.
Bake the cheesecake in the water bath in the preheated oven for 45 minutes.
Turn the oven off and leave the cheesecake in the oven with oven door slightly ajar for another 45 minutes.
Remove from oven. Run a knife along the edge of the cheesecake to loosen from tin. Chill in refrigerator at least 4 hours.