To prepare cupcakes, place muffin cup liners in 12 muffin cups. Coat each paper liner with cooking spray.
Weigh or lightly spoon flour into a dry measuring cup; level with a knife. Combine flour and next 3 ingredients in a small bowl, stirring with a whisk.
Combine 1/4 cup granulated sugar and banana in a small bowl; set aside. Combine remaining 1/2 cup granulated sugar, butter, and vanilla in a large bowl; beat with a mixer at high speed until well blended. Add eggs, 1 at a time, beating well after each addition. Add banana mixture to sugar mixture, beating well. Add flour mixture and yogurt alternately to sugar mixture, beginning and ending with flour mixture.
Spoon batter evenly into prepared muffin cups. Bake at 350° for 24 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 10 minutes on a wire rack, and remove cupcakes from pan. Cool completely on a wire rack.
To prepare frosting, combine cream cheese and vanilla in a large bowl; beat with a mixer at medium speed until smooth. Gradually add powdered sugar, beating just until smooth. Spread about 1 tablespoon frosting onto each cupcake. Sprinkle each cupcake with toasted walnuts, if desired.