Put the flour, eggs and sugar into a bowl. Pour in the milk and whisk until you have a smooth batter, then divide in half. To one half of the batter, whisk in the vanilla extract, and to the other half, whisk in the cocoa powder.
Lightly oil a non-stick pan, set over a medium heat. Using two spoons, alternately drop the white and dark batter on top of each other, a little off centre, so that the colours very slightly spread until you have 4 concentric circles. Cook until the underside is bubbly then flip and cook for 30 seconds more. Repeat the process with the rest of the batter. Serve drizzled with chocolate sauce.