Wash, peel and cut your beet into smaller cubes. Boil with some water until cooked and soft. Strain and blend the cooked beet into a smooth puree. Keep aside
In a double boiler, melt the chocolate and butter together on slow heat. make sure that the chocolate doesn't burn and turn off the stove once melted.
Preheat the oven to 350° Fahrenheit/ 180° Celsius.
In the meantime, while keeping an eye on the melting chocolate, add flour salt, 3/4 of the powdered sugar and baking powder to a mixing bowl. Separate the yolks from the whites and add the yolks to the mixing bowl with the flour etc. Combine the whole content, then mix in the beet puree too and at the end the slightly not too hot melted chocolate butter mixture. Combine the whole content to a smooth batter.
Beat the egg whites and when they are half way stiff, add the remaining powdered sugar and continue to beat to a stiff snow. First fold half of the stiff snow into the beet flour chocolate batter from earlier and then add the remaining half as well. Fold in carefully and don't mix in a turning motion, the folding helps to retain the fluffiness of the egg whites. Make sure that it is well combined into a smooth batter.
Spread some butter into the mold and dust with some flour, so that the cake won't stick after baking, and pour in the batter. I used two smaller square molds for the whole batter because I didn't have a bigger round mold. Best is to use a bigger round baking mold.
Bake at 350° Fahrenheit/ 180° Celsius. If you use smaller molds bake for 35-40 minutes, if you use a bigger mold bake for around 45 minutes. Test the cake after baking by poking into the center with a toothpick. If the toothpick returns gooey, the cake needs some more baking time.
If you want to cover the cake after baking with icing sugar then mix powdered (icing) sugar and a few drops of water into a smooth liquid paste and cover the cake with it.