Butter the insides of four ramekins well, and dust with flour. Set aside.
Melt butter and chocolate in the microwave and set aside to cool.
Whisk together egg yolks, eggs and sugar till light and creamy.
Add the chocolate mixture and vanilla extract and mix well.
Sift cocoa powder and flour in the mixture and stir to combine
Divide the mixture into the four ramekins and refrigerate for 30 minutes and up to two hours.
When ready to bake, pre heat the oven to 425F.
Place the ramekins in a casserole dish. Pour hot water into the dish, such that it comes halfway up the ramekins. Bake for 15-18 minutes. Once baked, let them cool for 10 minutes before inverting the ramekins on a serving plate to serve.