Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour and next 5 ingredients (flour through salt) in a large bowl; make a well in center of mixture. Combine buttermilk, oil, and egg whites; stir well with a whisk. Add corn, spinach, and jalapeño; stir well. Add to flour mixture, stirring just until moist.
Spoon batter into an 8-inch round cake pan coated with cooking spray. Bake at 375° for 35 minutes or until a wooden pick inserted in center comes out clean. Cool 10 minutes in pan on a wire rack; remove from pan. Cool completely on wire rack. Cut into 9 wedges. Serve with Mango Salsa.