Prepare a 20cm round pie dish by greasing with butter or non dairy alternative. Cream butter, sugar and vanilla. Gradually add egg to creamed mixture, beating well after each addition. Sift gluten-free flour, baking powder, cornflour and custard mix into creamed mixture. Mix to a firm dough. Turn out onto a lightly floured (gluten free) board and knead lightly. Roll out to 0.5cm thickness. Cut sections of pastry. Carefully lift pastry into prepared dish. Press down into dish.
Place into a medium hot oven and bake for approximately 10 minutes or until pale golden brown. While pastry base is cooking, make up filling.
Place all the ingredients into the bowl of an electric food blender. Blend until smooth. Carefully pour filling into pastry base in dish and then sprinkle with nutmeg. Place into oven and bake for approximately 45 minutes or until set in the centre. Remove from oven and allow to cool.