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Tip the sultanas into a bowl, add the Disaronno and set aside to soak for 1 hr. Heat oven to 180C/160C fan/gas 4 and lightly grease a 23cm loose-bottomed round cake tin. Melt the butter in a pan over a gentle heat and stir in the biscuit crumbs. Mix well, tip into the base of the tin and push down to compress it. Bake for 5 mins, then remove from the oven and allow to cool.
Meanwhile, beat the cheese with the eggs, soured cream, flour, vanilla and caster sugar – preferably with an electric whisk so it is nice and smooth. Stir in the sultanas (and any Disaronno left in the bowl), then pour onto the base. Bake for 30 mins until set but still with a slight wobble in the centre. Cool in the tin. Can be made 2 days ahead and chilled.
To serve, carefully turn out, dust with icing sugar and top with some broken amaretti biscuits. Serve with chilled glasses of Disaronno, if you like.