Whisk together the flours, sugar, and salt.
Beat the egg separately with a fork, add to the mixture, add the butter and knead until combined.
Wrap the dough in plastic cling film and refrigerate for 30 minutes .
Grease and flour a 9” tart crust.
Remove the dough from the fridge and roll it out into a circle on a lightly floured surface. Transfer the dough to the prepared pan, pressing it in on all sides. Poke holes with a fork.
Sprinkle the sugar evenly on top.