Butter and flour 9 X 5 loaf pan.

Sift flour, baking powder, baking soda, and salt together in a medium bowl. Set aside.

Using a large bowl and electric mixer, cream butter and sugar. Add yolks one at a time, beating well after each. Add sour cream, orange juice, lemon zest, and orange zest. Beat slowly until dissolving ribbon of batter forms when beaters are lifted. Stir in flour mixture.

Using clean, dry beaters and medium clean bowl, beat egg whites with cream of tartar until they are stiff, but not dry. Gently fold half of whites into cake batter. Fold remaining whites in, attempting to keep as much volume as possible.

Pour into prepared loaf pan. Bake until tester inserted in middle comes out clean, about 45 minutes.

Let cool in pan at room temperature for 15 minutes. Run knife carefully around cake to loosen; invert onto serving platter.

Cook all ingredients in small pan over low heat until sugar dissolves, whisking occasionally.

Increase heat to medium and boil until syrupy.

Pierce top of cake in several places. Pour syrup over.

Garnish with orange and lemon slices.