Line the base of a 22cm springform or loose-base flan tin with baking paper. Process biscuits until finely crushed. Add butter and process until combined. Press biscuit mixture over base and 3 1/2cm up the side of prepared tin. Place on a tray and refrigerate.
Peel, chop and puree 2 mangoes, you should have 1 1/2 cups.
Combine cream, buttermilk and sugar in a medium saucepan. Sprinkle over the gelatine. Place over medium-low heat. Gently whisk, with a silicon balloon whisk, for about 6 minutes until hot, do not allow to boil. Remove from heat. Add the white chocolate, stir until smooth. Stir in the mango puree. Strain into a large jug and cool for 30 minutes at room temperature. Pour into the tin and refrigerate for 6 hours or until firm.
To serve, remove the tart onto a serving plate or board. Cut the cheeks from the remaining mangoes. Using a large spoon, remove the mango fruit from the cheeks, slice. Decorate the tart with mango and berries. Cut into wedges, serve with remaining fruit.