Melt 4 tablespoons of butter in a small saucepan over low heat. With a brush, paint the inside of 6 ramekins with butter. Next, coat the inside of each with the sugar and set aside.
Put the remaining butter and chocolate chips in a large mixing bowl. Set the bowl on a saucepan filled with boiling water, making sure not to touch the water (1 inch of water). With a rubber spatula stir the butter and chocolate together until completely melted and smooth. Turn off the heat and set aside.
In a bowl combine the eggs, yolks, sugar and vanilla and mix (either hand mixer or stand mixer). Beat the egg mixture until it doubles in size and gets thick and a pale yellow.
Gently whisk the melted chocolate and butter into the egg mixture. Sift in the flour and gently mix. Pour the batter into the individual ramekins. Arrange them on a baking sheet and bake for 14 minutes. Remove the cakes from the oven and let rest for 3-4 minutes. Top with sifted powdered sugar and enjoy!