• First, melt half of the butter in a saucepan. Add the oreo cookies to a food processor (whole cookies including the white filling). Blitz the cookies to a fine crumb whilst adding the melted butter.
• Add the crumbs to a lined cake tin (8.5 inch/22cm) and press it firmly with the back of a metal spoon to create a smooth, even layer. Place it in the fridge while you make the filling.
• Cut the marshamallows into small pieces and brake up the chocolate into small chunks. Add them both to a large saucepan with the espresso, water and remaining half of the butter (cut into cubes).
• Melt the mixture over a low/medium heat and stir the mixture until everything has completely melted. When there are only a few lumps of marshmallows left, turn the heat off and add the mascarpone. 
• Stir the marscarpone into the chocolate mixture until combined and add a tiny pinch of sea salt.
• If there are still a few lumps of marshmallows left then turn the heat back on for a few seconds and stir until melted.
• Remove the chocolate cheesecake base from the fridge and pour the chocolate filling into the tin. Place it back in the fridge to set for at least 1 hour (preferably 2-3). Dust with unsweetened cocoa powder before serving (optional).

For topping (optional)

• Add the ricotta to a mixing bowl and whisk until smooth and creamy. Add the sugar and sieve in the cocoa powder. Mix everything together, taste and add more sugar if necessary.
• Add the mixture to a piping bag with a large star shaped nozzle. Pipe the frosting around the edge of the cheesecake and serve.