Set up a double boiler by putting a large stainless steel mixingbowl over a pot of simmering water. You can buy a double boiler,but it is just as easy to set one up using the pans you already own. Just make sure that no water can splash up and get into the chocolate. (You can see examples of double boilers or double boiler inserts at Cookware under double boilers).
Melt the butter and chocolate in the double boiler using a spatula to keep the chocolate from burning while you are combining the two ingredients. You want the consistency at the end to be silky.
While the chocolate is melting, whip the sugar, eggs and egg yolks together in a kitchen aid mixer until it triples in volume. For us, it was when the mixture reached the top and was just about to overflow the mixing bowl. I guess you can eyeball what three times the volume would look like if you were cutting this recipe in half.
If you don't have kitchen aid mixer, try using an electric hand mixer but it is going to make the next steps harder without having your hands free. So this is a great recipe to make with your spouse or even your kids.
Once the eggs and sugar reach triple volume, slowly fold in the melted chocolate using the spatula to scrape any chocolate sticking to the sides. On the kitchen aid mixer we were using, I had it running at almost full speed while the eggs and sugar were mixing but turned it down to 2 when I folded in the chocolate.
With the mixer still running at a slower speed, fold in the sifted flour into the chocolate and egg mixture and immediately stop the machine. Lower and remove the mixing bowl, remove the mixing whisk and stir the mixture by hand to make sure to scrape along the sides to be sure everything is well combined. Be careful not to over mix at this point.
Pour the mixture into the buttered and floured tins and bake at 350 degrees for about 5 minutes. The sides should be set but the centers should still be quite wiggly.
When done, remove from oven and gently remove the cakes from the tins by turning them over and gently taping the sides and bottom of the tins onto the serving plate. Sprinkle a little powdered sugar over the top and serve.
We didn't cook them right away so Chef Jeff covered and put them into the freezer until about 10 minutes before we were going to bake them. If you hold them for any length of time, you may want to remove them 30 minutes before baking.