For the base, crush the biscuits in a bag with a rolling pin, or pulse in a food processor. Add the butter and stir. Line a 20 x 30cm cake tin with non-stick baking parchment. Tip the biscuit mixture in and firmly press down. Chill.
Mix the passion fruit flesh with the juice. Place the gelatine in a small bowl of cold water to soak. Pour the flesh and juice into a saucepan and heat until hot. Squeeze the water out of the gelatine, add to the pan and stir until dissolved, then remove and pour into a bowl.
Add the milk and loosely whipped cream and fold together. Pour into the tin and chill until firm, about 2 hrs.
For the coulis, purée 1 mango with the lime juice and sugar in a food processor, then stir to dissolve the sugar. Chop up the other mango.
To serve, cut the traybake into squares. Top each one with the coulis, some chopped mango and passion fruit.