Combine first 5 ingredients in a large bowl; beat with a mixer at medium speed until smooth.
Lightly spoon flour into dry measuring cups, and level with a knife. Combine flour, cream of tartar, baking soda, and salt in a bowl, stirring with a whisk. Add the flour mixture to banana mixture, stirring just until moist. Spoon the batter into an 8 x 4-inch loaf pan coated with cooking spray.
Bake at 350° for 40 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 rack.
Note: To freeze individual slices, place in heavy-duty zip-top plastic bags. Remove excess air from bags; seal and freeze for up to 1 month. To thaw, let stand at room temperature.