Preheat the oven to 180c. Line two baking trays with baking paper.
Beat the butter and sugar until light and creamy. Beat in the vanilla essence. Sift in the flour and custard powder and mix to a soft dough.
Roll level tablespoons of the mixture into 28 balls and place on the trays. Flatten slightly with a floured fork
Bake for 20 minutes, or until a light golden colour. Cool on a wire rack.
To make the filling, beat the butter and sugar until light and creamy, then beat in the passionfruit pulp. Use the filling to sandwich the biscuits together. Leave to firm before serving.