Preheat the oven to 180°C (350°F/Gas 4). In a saucepan, melt the butter, chocolate and cocoa powder over low heat. In a bowl, whisk the oil, sugar, vanilla, eggs and egg yolks vigorously for a few minutes. Combine the dry ingredients and add them to the whisked mixture, then stir in the melted mixture.
Grease and flour a 24 cm (9½ inch) square cake tin. Spread the batter in the tin. With a wet hand, flatten and smooth the surface. Bake for about 35 minutes. The top of the cake will become shiny and start to crack and it should be fairly solid to the touch. Allow to cool and cut into 16 squares.