Cream butter and sugar until light and fluffy; beat in egg yolks one at a time. Grate zest of orange and squeeze juice. Retain 1/2 teaspoon of zest and 1 tablespoon of juice. Add remaining zest and juice to creamed mixture with 1 cup of yogurt; beat well. Sift together flour, baking powder, baking soda and salt. Stir into creamed mixture, beating well. Beat egg whites until stiff. Fold into batter. Butter a 1 quart tube pan and flour lightly. Pour in batter. Bake in a preheated 350 degree oven for 45 to 50 minutes or until cake tester comes out clean. Cool for 10 minutes; invert on a wire rack. Sift powdered sugar into a small bowl. Mix with remaining zest juice and yogurt. Spread glaze over cooled cake and allow to harden slightly.