Preheat oven to 170c/325f Grease two 6-piece doughnut tins with vegetable oil or spray.
In a large bowl, mix the flours, sugar, baking powder, baking soda, and salt until combined. Add in the egg, yogurt, oil, milk and grapefruit juice and zest. Mix, using a spatula until everything is just combined. Don’t overmix, as this will toughen the doughnuts.
Spoon the doughnut batter into a piping bag (no tip required) or a plastic bag with the corner cup off. You want the hole to be approx. ¾”. This makes it easier to fill the doughnut tins. Pipe the batter into each tin.
Place in the oven and bake for 10-15 minutes, until an inserted skewer comes out clean.
Whilst the doughnuts are cooking, whisk together the grapefruit juice, zest and sugar and put to one side.
Take the doughnuts out of the oven, leave to cool for 10 minutes in the tray, then dip the doughnuts into the glaze (you may have to mix the glaze again before dipping). Turn over and place on a cooling rack. Sprinkle on the grapefruit zest.
Leave to cool on the cooling rack until the sugar glaze crisps up.