1 1/2 cups all-purpose flour
1/2 teaspoon salt
1/4 teaspoon baking powder
1/4 teaspoon baking soda
1 large orange
1 cup sugar
1/2 cup buttermilk, at room temperature
1/2 teaspoon vanilla extract
1/4 cup butter, softened (no substitutions)
1 large egg
2 large egg whites
2 tablespoons water
1/4 cup sugar
Preheat oven to 350*F. Lightly coat a 9×5-inch loaf pan with vegetable cooking spray. Sift together flour, salt, baking powder and baking soda in medium bowl; set aside.
With vegetable peeler remove peel from orange; coarsely chop. Cut orange in half and squeeze 1/2 cup juice. Process sugar and peel in food processor, pulsing until peel is finely chopped. Combine buttermilk, 1/4 cup orange juice and vanilla in a small glass measuring cup. (Reserve remaining juice for caramel sauce.)
Beat butter and orange-sugar mixture in large mixer bowl on medium-high speed until combined. Beat in egg and egg whites, one at a time, beating well after each addition. At low speed, add flour mixture alternately with buttermilk mixture, beginning and ending with dry ingredients. Spoon batter into prepared pan. Bake 35 to 40 minutes or until toothpick inserted in center comes out clean. Cool on wire rack 15 minutes; invert and remove pan.
Make Orange-Caramel Sauce: Meanwhile, combine water and sugar in small saucepan; stir with wooden spoon until sugar is moistened. Bring to simmer over medium-low heat; cover and simmer 2 to 3 minutes until sugar dissolves. Uncover and cook until sugar turns amber, 5 to 6 minutes. Remove from heat and slowly stir in reserved orange juice (mixture will bubble vigorously). Return to heat and cook, stirring, until sugar dissolves. Cool sauce and serve with cake.