Cut orange in half and extract juice with a hand juicer. Use the flesh and the juice. Grate the rind with a zester.
Put all ingredients (except flour and baking powder) into a bowl and beat on medium speed for 7 minutes, making sure to scrape the side of the bowl. Sift flour and baking powder, add to wet ingredients beat 2 more minutes.
Pour into a greased and floured 9" or 10"spring form pan, with a hole in the center, and a design on the bottom. I wouldn't recommend a cake pan without a hole in the center, as the cake may fall in the middle.
Bake at 350°F for 30-40 minutes or until knife inserted in cake comes out clean. It's ok if the top starts to crack. You'll flip it over when it's done, so the design is on the top.
Sprinkle with icing sugar and grated orange peel or mix some orange juice and icing sugar to make a glaze.