Whip the egg whites to make a meringue. Add the half of the sugar and beat again. Pour the meringue in a bowl and leave aside. Beat the egg yolks with the remaining sugar very well.
Melt in a double boiler (bain marie) the chocolate with the butter and add the vanilla. Carefully mix the chocolate with the the yolk mixture, then add the flour and keep last the meringue. Stir gently with a spatula.
Brush with butter a medium-size baking dish (with relative high walls). Pour the dough inside and bake in preheated oven at 180 ° for 20 minutes. (this chocolate pie is nice to stay damp inside). Let it cool slightly, sprinkle with cocoa and serve plain or with vanilla ice cream.