Preheat to 350˚F.
You will need an ungreased 10-inch tube cake pan with a removable bottom or angel food pan.
(Do not use a nonstick surface.)
Sift the flour, ½ cup sugar, the baking powder and salt into a medium-sized bowl.
Make a well in center and add the melted butter, orange zest, orange juice, egg yolks and vanilla. Beat until smooth, 1 to 2 minutes.
In a mixer, whip egg whites and cream of tartar to soft peaks.
Slowly beat in the remaining ½ cup sugar and continue to beat until stiff glossy peaks form.
Stir a large blob of whites into the batter.
Then fold in the remaining egg whites until even in color.
Spread in pan and bake 1 hour. Note-mine was done in about 30 minutes so watch it closely! Immediately invert and let the cake hang upside down until completely cool.
Turn right side up and run a sharp knife around the edge to separate the cake from the pan. Remove sides.
Run a knife around the center tube and under the cake; remove the pan bottom.
Turn the cake right side up on your serving plate.
