- Preheat the oven to 400°F (200°C) and grease 2 baked doughnut pans (or use one in batches).
- Whisk the sugar and lemon zest together in a large mixing bowl (to draw out the flavour in the zest). Whisk in the buttermilk, oil, eggs and vanilla.
- In a separate bowl, sift the flour, cornstarch, baking powder and salt. Ad this all at once to the liquid and whisk until well-combined (the batter will be fluid).
- Transfer the batter to a pitcher and slowly pour the batter into each doughnut cup, filling ¾ full. Bake for 8 to 10 minutes until the doughnuts spring back to the touch, but there is no browning. Cool the doughnuts for 10 minutes in the pan and prepare the sugar for rolling as you wait.
- For coating, whisk the sugar and lemon zest together. Tip the doughnuts out of the pan and, while still warm, roll them in the lemon sugar and set on a rack to finish cooling.
The doughnuts are best enjoyed the day they are baked, but will keep in an airtight container for up to 2 days.