Preheat the oven to 180°C (fan 160°C) and grease your donut pan generously. Use a pastry brush to coat each cavity with melted butter or a neutral oil. The pan should look glossy but not pooled. This step ensures your donuts release cleanly with that perfect golden edge.

Whisk the dry ingredients together in a large mixing bowl. Combine the flour, caster sugar, baking powder, bicarbonate of soda, cinnamon, and salt. Whisk until the mixture is uniform and pale beige in colour, with no streaks of baking powder visible. The cinnamon will smell warm and slightly sweet as you work.

In a separate jug, whisk the wet ingredients. Beat the buttermilk, egg, melted butter, and vanilla extract together until the egg is fully incorporated and the mixture looks pale yellow and slightly frothy on top. The butter should be cool enough that it doesn’t scramble the egg—test a drop on your wrist; it should feel just warm, not hot.

Pour the wet mixture into the dry ingredients and stir gently. Use a rubber spatula and fold just until the flour disappears. The batter will look thick, lumpy, and slightly shaggy—that’s exactly right. Stop mixing the moment you no longer see white flour. Overmixing will make the donuts tough and dense.

Toss the blueberries with the reserved tablespoon of flour, then fold them into the batter. Gently turn the berries into the mixture using only three or four folds. You want the blueberries scattered throughout, but the batter should still be thick and slightly sticky. A few purple streaks are fine; they’ll bake out.

Spoon the batter into the prepared donut pan. Fill each cavity about three-quarters full—the batter will spread and rise as it bakes. A piping bag makes this step incredibly neat, but two spoons work just as well. The surface should look bumpy from the berries peeking through.

Bake for 10 to 12 minutes, or until the donuts are golden brown. The tops will spring back when you press them lightly with your finger, and a skewer inserted into the thickest part should come out with just a few moist crumbs attached. The kitchen will smell like a bakery—warm butter, vanilla, and fruity sweetness.

Let the donuts cool in the pan for 3 minutes, then transfer them to a wire rack. Run a small offset spatula or butter knife around the edges if they feel stuck. The donuts should lift out easily, revealing a deeply golden ring with tiny dark blueberry flecks. Let them cool completely if you plan to glaze them, or serve them warm as they are.