Preheat the oven to 350 degrees F (175 degrees C). Prepare an ungreased 10-inch tube pan with a removable bottom.

In a medium bowl, sift together the flour, 1/2 cup sugar, baking powder, and salt. Make a well in the center and add melted butter, orange juice, orange zest, egg yolks, and vanilla extract. Beat with an electric mixer for 1 to 2 minutes until well combined.

In a separate bowl, beat the egg whites and cream of tartar until foamy. Gradually add the remaining 1/2 cup sugar, continuing to beat until stiff peaks form.

Fold 1/3 of the beaten egg whites into the batter, then gently fold in the remaining whites until no streaks remain.

Pour the batter into the ungreased tube pan. Bake in the preheated oven for 60 to 75 minutes, or until the top springs back when lightly touched and a toothpick inserted into the center comes out clean.

Immediately invert the pan and let the cake hang upside down until completely cool.

Once cooled, turn the cake right side up and run a sharp knife around the edge to separate the cake from the pan. Remove the sides of the pan and carefully run a knife around the center tube to release the cake. Place the cake right side up on a serving plate.

To make the glaze, combine the confectioners' sugar, orange juice, and orange zest in a small bowl. Spread the glaze over the top of the cake, allowing some to drip down the sides.

Let the cake stand at room temperature until the glaze has set, about 1 hour.

Slice and serve your delicious Orange Fluff Cake!