1. Position a rack in the center of the oven and preheat to 350 degrees F. Have ready an ungreased 10-inch tube pan with a removable bottom. (Do not use a tube pan with a nonstick surface.)

2. Sift the flour, half cup sugar, the baking powder, and salt into a medium-sized bowl. Form a well in the center and add the melted butter, orange zest, orange juice, and the yolks, and vanilla. With an electric mixer, beat until smooth, 1 to 2 minutes.

3. In a large bowl combine the egg whites and cream of tartar; beat with an electric mixer until soft peaks begin to form. Gradually beat in the remaining half cup sugar and continue to beat until stiff glossy peaks form. Stir a large spoonful of the whites into the batter to lighten. With a spatula, quickly fold in the remaining egg whites, half at a time, to make a fluffy batter that is even in color. Turn into the ungreased tube pan and bake one hour to one hour and 15 minutes, until the top springs back when lightly touched and a toothpick inserted emerges without crumbs clinging to it. Immediately invert and let the cake hang upside down until completely cool. (If the pan does not have tabs on it's rim, place tube over the neck of a bottle). 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 a serving plate.

4. Prepare glaze: In a small bowl combine the confectioner's sugar, orange zest, and orange juice to make a glaze. Spread over the top of the cake, letting some drip down the sides. Let stand at room temperature until glaze has set, about one hour.