Place the butter in a small saucepan and over very low heat. Stir until almost melted, and then start adding the chocolate; continue stirring until almost all melted. Remove from heat and stir until chocolate is completely melted. If you don't follow these directions, you may end up with hard chocolate. Set aside. (This is one of the steps that makes this recipe easier--no double boiler. However, if you're not comfortable melting the butter and chocolate in a pot, feel free to do this step in a double boiler.)

In a medium bowl, beat egg whites until soft peaks form, then add ⅓ of the sugar, beating until stiff peaks form.

In a larger bowl, using the same beaters, beat the egg yolks and remaining sugar until very thick and pale yellow. This will only take a few minutes on a stand mixer, but much longer by hand. Add the melted butter and chocolate mixture, and beat on low speed, until evenly combined.

Add cocoa powder (or flour, if using) and continue beating at low speed. Clean the side of the bowl with a spatula occasionally.

If using a mixer, remove the bowl, and with the spatula, gently fold about one third of the beaten egg whites into the mixture. Do not stir or beat the mixture, as you want to keep the air in the batter. Continue adding the egg whites and folding, one third at a time until it is evenly mixed.

Gently spoon the mixture into the prepared pan, and spread evenly. If there is a little unmixed egg white here and there, it's okay.

Place into the preheated oven and bake for approximately 35 to 40 minutes. Torte will rise and crack as it bakes. It will be ready when a toothpick or cake tester inserted in the center comes out wet, but not runny.

Let cool completely in the pan, placed on cooling rack. The torte will fall as it cools and it is supposed to do this.

Run a thin knife along the inside of the pan, then remove the side of springform pan, and set the flourless chocolate torte on a serving plate.

Dust with powdered sugar if desired.