Preheat oven to 170 degrees Celsius  and grease a 20 cm tart dish.

Melt chocolate and butter in a glass bowl over a double boiler, stirring occasionally. 
Whisk together the eggs, sugar and vanilla essence until a ribbon effect occurs (usually pale and tripled in volume). Add in cocoa and mix well. 

Slowly beat in the melted chocolate and pour mixture into dish.

Bake for 20-25 minutes then take out the oven and leave to cool for 15 minutes, sprinkle with cocoa powder and serve

Delicious served plain or with ice cream/whipped cream