Preheat oven to 350°. Prepare cake mix according to package directions, adding 1 teaspoon vanilla and salt before mixing batter. Transfer to a greased 13x9-in. baking pan. Bake and cool completely as package directs., Whisk peanut butter and milk until blended. Using the end of a wooden spoon handle, poke holes in cake 2 in. apart. Slowly pour 2 cups peanut butter mixture over cake, filling each hole. Refrigerate cake and remaining peanut butter mixture, covered, until cake is cold, 2-3 hours., Combine remaining vanilla and remaining peanut butter mixture; gradually beat in enough confectioners' sugar to reach a spreading consistency. Spread over cake. Add toppings as desired. Refrigerate leftovers.