Measure ingredients as detailed above. Grease 20 x 12 x 5 cm cake pan with butter. Line pan with parchment paper. Generously grease paper. Preheat oven to 170°C.

Heat oil till very hot, almost smoking. Add to cocoa powder. Mix thoroughly. (There should be a chocolate aroma. If there isn't, either your oil isn't hot enough or you've got crap cocoa powder, or both.) Let mixture cool down slightly, till bowl doesn't feel hot but is still quite warm. Add milk. Stir thoroughly. (You should now have a smooth or slightly grainy paste that coats sides of bowl thickly.) Add vanilla extract and salt. Stir again. Set aside to cool down completely.

Whisk glucose, eggs and yolk till starting to thicken. Gradually add castor sugar whilst still whisking. Continue to whisk till mixture is thick and pale, reducing speed towards the end to remove big air bubbles.

Add cocoa paste in 2 batches. Whisk till you don't see any streaks of cocoa after each addition. Thoroughly scrape bottom of bowl with spatula and fold till evenly mixed.

Sift cake flour and baking soda into bowl in 2 batches. Mix with whisk after each addition, stirring just top part of egg mixture. Scrape down and fold with spatula till just evenly mixed, banging mixing bowl against worktop 2-3 times.

Pour batter into cake pan. Tap pan against worktop 2-3 times.

Bake in bottom of oven till batter doesn't move when gently jiggled, about 20 minutes, rotating pan as necessary so that cake rises evenly. Transfer to middle of oven. Continue baking till skewer inserted into centre of cake comes out clean, 5 minutes or so.

Remove cake to wire rack. Wait a few moments. When top of cake starts to wrinkle, unmould. Peel paper from sides of cake. Leave cake on wire rack to cool down. Cut with serrated knife. Remove paper from bottom of cake. Serve.