Grease and line a 20cm springform tin with baking parchment. Mix the crushed digestive biscuits, melted butter, and demerara sugar until combined. Press firmly into the base of the tin and chill for 20 minutes.
Melt the white chocolate gently over simmering water or in short microwave bursts. Leave to cool slightly.
Beat the mascarpone, cream cheese, icing sugar, vanilla extract, and double cream until smooth, light, and fluffy.
Fold the cooled melted white chocolate into the cream cheese mixture until fully incorporated.
Blend half of the raspberries with the icing sugar until smooth, then strain through a sieve to make a seedless raspberry coulis.
Spread half of the cheesecake filling over the chilled biscuit base. Pipe or spoon some raspberry coulis into the filling, then add the remaining cheesecake mixture and gently swirl more coulis through the top.
Cover loosely with cling film and refrigerate for at least 6 hours, preferably overnight.
Carefully remove the cheesecake from the tin. Decorate with fresh raspberries, grated white chocolate, raspberry coulis, and mint leaves before serving.