Melt the butter. Crush the Digestive biscuits and stir in the butter. Spread over the bottom of a springform pan lined with baking paper. Press evenly down and transfer to the fridge to chill.
Combine blueberries and granulated sugar and set on medium heat. Bring to a boil, stir from time to time and cook for 3-4 minutes. Set aside from the heat and blend with an immersion blender.
Heat 2/3 of the whipping cream (in the microwave or on the stove) and stir in the white chocolate until melted.
Soak the gelatin sheets in cold water for 2 minutes. Warm 1/3 of the whipping cream, squeeze the water from the gelatin and add softened gelatin to the hot cream. Stir to dissolve.
Combine cream cheese and powdered sugar and whip with an electric mixer for 1 minute. Pour the gelatin mixture in and mix until combined. Divide mixture equally between 2 bowls.
Pour the white chocolate mixture into one of the bowls and mix for 4-5 minutes. It will not whip to peaks, but mixing incorporates air.
Reserve 3-4 tbsp of the blueberry purée for the topping and pour the rest of the blueberry mixture into the other bowl with cream cheese and mix until combined.
Spread blueberry mixture over the chilled biscuit base, making sure it reaches all the way to the sides of the pan.
Slowly pour the white chocolate mix over the blueberry layer. Both mixtures are quite runny, so I recommend pouring the next one over a spatula or a spoon and not directly onto the blueberry layer to avoid mixing.
Transfer the reserved blueberry purée to a piping bag and pour it over the white chocolate layer. Using a chopstick draw perpendicular lines to create a swirling pattern. top with fresh blueberries and extra chocolate if desired.
Place the cake in the fridge and let set for a minimum of 4 hours.