Boil the kettle and prepare your slow cooker (at least 22cm wide). Make a trivet for your cheesecake by scrunching a long piece of foil into a sausage. Roll into a loose coil and put on the bottom of the slow cooker. Turn the slow cooker to high and pour in enough hot water to come 4 cm up the sides. Wrap the outside of a 20cm springform cake tin in two layers of cling film and then foil – make sure there are no gaps for water to seep in. Grease the inside with butter, line with cling film and grease again, then line the base and sides with baking parchment.
Tip the biscuits and 50g hazelnuts into a food processer and blitz to fine crumbs. Add the butter and blend again until well combined. Tip the crumbs into the tin – use a spoon to press into the base. Chill for 10 mins.
In a saucepan, heat the muscovado and milk until the sugar has dissolved. Set aside to cool. In a bowl, beat the cream cheese, flour, vanilla and eggs until smooth. Stir in the soured cream, Frangelico (if using) and cooled sugar mixture until well combined. Pour into the tin and carefully put it in the slow cooker. Wrap the lid in a tea towel to prevent condensation from dripping onto the cheesecake. Cover and cook for 2 hrs, then turn off the slow cooker. Leave the cheesecake inside without opening for another 2 hrs. Remove from the slow cooker and cool at room temperature for a further 1 hr, then chill for 4 hrs, or overnight.
Put the cornflour, caster sugar and half the blackberries in a saucepan and set over a high heat. Cook for 3-4 mins, squashing the blackberries a little, until syrupy. Toss in the remaining berries, heat through, then remove from the heat and cool until you’re ready to serve. Roughly chop the remaining nuts and toast in a frying pan.
To serve, carefully remove the cheesecake from the tin, remove the baking parchment and transfer to a plate or cake stand. Top with the berries and hazelnuts, and serve with a shot of Frangelico, if you like.