Using a food processor (or a Ziploc bag and rolling pin), grind/process the sweet biscuits to powder and pour the melted butter. Process until it resembles coarse wet sand. If too dry, add a tablespoon of melted butter. Set aside.
Preheat the oven to 135°C (the low temperature helps keep the cheesecakes smooth and creamy). Prepare a water bath (I use hot water), to be placed under the cupcake tray (not touching the cupcake tray directly) to steam the cheesecake cupcakes in the oven.
Line standard cupcake tins with paper/aluminum liners. Place 1 tbsp of crumbs in the bottom of each liner and press evenly to create the biscuit base. Place in the fridge until ready to use.
In a large bowl use an electric mixer to beat the cream cheese and mascarpone until smooth and combined. Gradually add the sugar and beat until combined. Beat in the vanilla. Add the eggs, one at a time, until combined. Beat in the sour cream and salt. Reserve a cup of the mixture before adding the ubeness to the rest of the batter, to create the white swirls on the cheesecake cupcakes later on.
Divide the ube-flavored batter evenly among the lined cups, filling each 2/3 of the way. Place a teaspoon of the reserved cheesecake mixture on top and using a toothpick, create swirls. Careful not to overmix to keep a white and purple swirl design, and not to touch the biscuit base.
Bake for about 22 - 25 minutes at 135°C, with a hot water-bath under the cupcake tray (not touching the tray but directly below it), until the filling is set. Transfer the baked cheesecake cupcakes to wire racks to cool completely. Refrigerate for at least 4 hours or overnight before serving.