Preheat oven to 160°C Fan. Line the base of a 23cm springform tin.
Blitz the Oreo cookies into crumbs, then stir through melted butter. Press firmly into the tin. Chill while you make the filling.
Pour the Guinness into a small pan and simmer until reduced to about 80ml. Remove from heat, add chopped chocolate, and stir until melted and glossy. Let cool slightly.
In a stand mixer, beat cream cheese, mascarpone, and sugars until smooth. Add eggs one at a time, followed by vanilla, flour, and salt. Finally, whisk in the Guinness-chocolate mixture until fully combined.
Pour into the prepared tin, tap to release any bubbles, and place the tin into a water bath (wrap it tightly with foil so no water leaks in). Bake 90-95 minutes until the edges are set but the centre still has a wobble. Chill overnight in the fridge.
Whip all the topping ingredients together until very thick and spoonable. Spread generously over the chilled cheesecake.
