Preheat and prep your pan: Set your oven to 180°C (350°F) and lightly grease a 12-cavity donut pan with a little melted butter or a spritz of non-stick spray. You want the pan ready so the batter doesn't sit around waiting.

Whisk the dry ingredients together: In a large mixing bowl, combine the flour, 150g caster sugar, baking powder, bicarbonate of soda, 1 teaspoon cinnamon, and salt. Whisk them thoroughly — you'll see the pale beige powder become an even, sandy colour, and you'll smell the warm spice rising from the bowl.

Mix the wet ingredients separately: In a smaller bowl or a large jug, whisk the egg until it's a uniform pale yellow and slightly frothy on top. Pour in the buttermilk, the cooled melted butter, and the vanilla extract. Whisk again until everything looks like a single, creamy liquid with no streaks of egg white.

Combine wet and dry: Pour the wet mixture into the bowl of dry ingredients. Stir with a spatula or a wooden spoon, using a folding motion. Stop the moment you see no more dry flour — the batter should look thick, slightly lumpy, and glossy. Overmixing will make the donuts tough, so as soon as the flour disappears, you're done.

Fill the donut pan: Spoon the batter into a piping bag (or a sturdy zip-lock bag with the corner snipped off) and pipe it into the prepared cavities, filling each one about ¾ full. The batter should sit just below the rim. You'll see the batter settle slightly as it fills the grooves.

Bake until golden and springy: Place the pan in the oven and bake for 8 to 10 minutes. At the 8-minute mark, check them: the tops should be a light golden brown, and when you gently press the surface of a donut with your fingertip, it should spring back immediately. A skewer inserted into the thickest part should come out clean, with perhaps a few moist crumbs clinging to it.

Cool briefly, then coat: Let the donuts rest in the pan for exactly 2 minutes. Then, turn them out onto a wire rack. While they are still warm but not piping hot — you should be able to hold one comfortably — dip each donut into the bowl of melted butter, letting the excess drip off for a second. Immediately toss it in the bowl of cinnamon sugar, making sure every nook and cranny is covered. You'll hear a faint sizzle as the hot donut hits the butter, and the sugar will stick immediately, forming that signature crackly crust.

Serve or store: Place the coated donuts on a plate or cooling rack. They are at their absolute best within an hour of being made, when the exterior is still crisp and the inside is tender and soft.