1 Position a rack in the center of the oven and preheat to 350 degrees F (175 degrees C) . Have ready an un-greased 10 inch tube pan with a removable bottom. Don’t use a nonstick surface pan.
2 Sift the flour, 1/2 cup sugar, baking powder and salt into a medium bowl. Form a well in the center and add the melted butter, 1/3 cup orange juice, 1 teaspoon orange zest, egg yolks and vanilla. Beat for 1 to 2 minutes with an electric mixer.
3 In a large glass or metal mixing bowl, beat egg whites and cream of tartar until foamy. Gradually add 1/2 cup sugar, continuing to beat until stiff peaks form. Fold 1/3 of the whites into the batter, then quickly fold in remaining whites until no streaks remain.
4 Pour batter into the un-greased tube pan. Bake for 60 to 75 minutes, until the top springs back when lightly touched and a toothpick inserted comes out clean. Immediately invert and let the cake hang upside down until completely cool.
5 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.
6 To make the glaze: In a small bowl combine the confectioner’s sugar, 2 tablespoons orange juice, and 2 teaspoons orange zest. 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.