Beat first 3 ingredients at medium speed with an electric mixer until blended; stir in chocolate morsels, walnuts, and 1/2 teaspoon vanilla. Set aside.
Beat remaining 1/2 teaspoon vanilla, butter, sugar, and 1/3 cup brown sugar until creamy. Add egg, and beat until blended.
Combine flour, soda, and 1/2 teaspoon salt; stir into butter mixture.
Place paper baking cups into miniature (1 3/4-inch) muffin pans, and coat with cooking spray; spoon 1 1/2 teaspoons batter into each cup.
Bake at 350° for 12 minutes. Spoon morsel mixture over cupcakes, and bake 10 more minutes.