Preheat your oven to 180°C (fan-assisted) and line a 20 cm square baking tray with parchment paper. You'll know the oven is ready when the heating element glows orange and you feel a steady wave of warmth when you open the door.

Whisk the dry ingredients together in a large bowl: plain flour, caster sugar, baking powder, bicarbonate of soda, and salt. The mixture should look pale and uniform, with no lumps of brown from the bicarbonate.

Add the wet ingredients to the same bowl: milk, melted butter, eggs, and vanilla extract. Stir with a wooden spoon until just combined—stop when you no longer see streaks of flour. The batter will be thick and slightly glossy, with a soft, drop-biscuit texture.

Make the cinnamon swirl in a small bowl: combine light brown sugar, ground cinnamon, and softened butter. Mash with a fork until the mixture resembles damp, sandy crumbs that clump together when pressed. You'll smell the warm, spicy fragrance of cinnamon as you work.

Assemble the cake by spreading half the batter into the prepared tray. It will be thick, so use a spatula to spread it evenly to the edges. Sprinkle the cinnamon mixture over the batter, then dollop the remaining batter on top. Use a knife to gently swirl the layers together—don't overmix, or the swirl will disappear into the cake. The surface should look marbled with dark brown ribbons.

Bake for 25–28 minutes, rotating the tray halfway through. The cake is ready when the top springs back when lightly pressed and a skewer inserted into the centre comes out with a few moist crumbs attached. The kitchen will smell like a bakery, with notes of toasted sugar and cinnamon.

Cool in the tray on a wire rack for 10 minutes. While the cake is still warm, prepare the glaze by whisking icing sugar and milk together until smooth. The glaze should be thick enough to coat the back of a spoon but still pourable. Drizzle it over the warm cake in zigzag lines, letting it sink into the swirls. The glaze will set into a soft, white sheen as it cools.

Slice and serve while still warm. The cake should feel tender and moist, with visible cinnamon ribbons running through each square.