In a large mixing bowl ... Break the egg into the base of a measuring jug and add the buttermilk to the 425ml (14fl oz/1 3/4 cup) line (the egg is part of the liquid measurement).

To make the crust: In a large mixing bowl, whisk together the flours, sugar, and salt; set aside In a large measuring cup or a small bowl, beat the egg with 1/3 cup of the water and the vinegar.