Preheat oven to 350 degrees or according to package (cake mix) instructions. Line a cupcake/muffin pan with cupcake liners and set aside.
In a large bowl combine the the cake mix and pudding mix and give it a quick stir to mix the dry ingredients together. Add in the eggs, sour cream, oil, milk and vanilla extract, mix everything together using a handheld mixer on medium speed, stop and scrape down the sides of the bowl as necessary. Mix until batter is thoroughly combined and smooth, do not over beat the cake batter. Scoop cake batter into cupcake pan, filling each liner about half way, do not fill to the top. Bake in the oven for 15 to 18 minutes, cupcakes are done when a toothpick inserted in the center comes out clean. Allow cupcakes to cool completely.
Using a cupcake corer or just a small spoon, scoop out a small hole from the top of the cupcake to the center of the cupcake. It should look like a small well. Spoon out a small amout of hot fudge sauce (room temperature, do not heat the hot fudge sauce) and fill the hole of the each cupcake.
Make the frosting: add the butter to a large mixing bowl, beat on medium speed for about 2 to 3 minutes. Scrape down the sides of the bowl and add in the powdered sugar, beat on low speed for about a minute and then work your way back up to medium speed. While on medium speed add in the milk, vanilla extract and the pinch of salt. Allow the frosting to mix for about 3 to 5 minutes on medium speed so it can get a nice fluffy texture. Frost cupcakes to your preference!
Garnish the top of each cupcake with a drizzle of hot fudge, chopped nuts, rainbow sprinkles and a cherry! You can also use cookie crumbs, candy pieces or whatever you would love on your cupcake sundae, enjoy :)