Grease a 40.5 x 26.5 x 5cm (16 x 10 x 2in) baking tin (or line with non-stick baking parchment), then sprinkle with flour and shake the tray to spread it evenly. Preheat the oven to 180°C/350°F/Gas 4.
Put the egg yolks into a bowl and add the oil, milk