Preheat oven to 160°C Fan and line the base of a 23cm springform tin
Blitz Oreo cookies into crumbs and stir through melted butter
Press the mixture firmly into the tin and chill
Pour Guinness into a small pan and simmer until reduced to about 80ml
Remove from heat, add chopped chocolate, and stir until melted and glossy, then 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
Whisk in the Guinness-chocolate mixture until fully combined
Pour into the prepared tin, tap to release bubbles, and place in a water bath wrapped tightly with foil
Bake 90-95 minutes until edges are set but centre still has a wobble
Chill overnight in the fridge
Whip all topping ingredients together until very thick and spoonable
Spread the topping generously over the chilled cheesecake
