Preheat your oven to 180°C (160°C fan) and line a 20cm square baking tin with a strip of baking paper, leaving an overhang on two sides for easy lifting. The paper should sit flush against the corners.

Whisk the dry ingredients together in a large mixing bowl. Add the flour, cocoa powder, caster sugar, baking powder, bicarbonate of soda, and salt. Whisk until everything looks evenly brown and no lumps of cocoa remain. The mixture should feel light and smell deeply of chocolate.

Add the wet ingredients — the oil, eggs, and vanilla extract — right into the centre of the dry mixture. Stir with a spatula or wooden spoon until a thick, dark paste forms. It will look quite stiff at this stage, and the scent of vanilla will start to cut through the cocoa.

Pour in the boiling water and stir gently but steadily. The batter will thin out dramatically as you mix, turning from a stiff paste into a smooth, runny liquid. Keep stirring until no streaks of dry flour remain and the batter has a glossy, silky sheen. Fold in the chocolate chips at this point if using; you’ll hear them clinking against the bowl as they disperse.

Pour the batter into the prepared tin. It will spread easily on its own — give the tin a gentle tap on the counter to release any air bubbles. The surface should look even and slightly rippled.

Bake for 22–25 minutes. At the 22-minute mark, insert a skewer into the centre. It should come out with a few moist crumbs clinging to it, not wet batter. The top will look set, and the edges will have begun to pull away slightly from the paper. A faint crackle on the surface is perfectly normal.

Cool in the tin for 10 minutes, then lift the cake out using the paper overhang and transfer to a wire rack. Allow it to cool completely before slicing. As it cools, the crumb will settle into that dense, fudgy texture we’re after.