Stir together 1/2 cup brown sugar, 1 egg, and 1/8 teaspoon salt in a medium bowl; stir in chocolate morsels, walnuts, and 1/2 teaspoon vanilla. Set aside.
Beat remaining 1/2 teaspoon vanilla, butter, 1/3 cup sugar, and 1/3 cup brown sugar at medium speed with an electric mixer until creamy. Add remaining egg, and beat until blended.
Combine flour, baking soda, and 1/2 teaspoon salt; stir into butter mixture until blended.
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 chocolate morsel mixture over cupcakes; bake 10 more minutes.