Preheat oven to 350F and line a 15x10x1 Jellyroll pan with parchment paper (or generously grease and flour the pan).
In KitchenAid cream butter on medium speed for 30 seconds.
Add sugar and cream together until light and fluffy (about 1 minute). Pause to scrape down the sides of the bowl, if needed.
Stir in eggs, one at a time.
Add vanilla extract and stir.
With mixer on low-speed, stir in sour cream until combined.
In separate bowl, whisk together flour, baking soda, and salt.
Measure out your milk and turn your mixer on medium-low speed
Gradually add some of your flour mixture (about ¼), allow it to stir until just combined and then add about ¼ of your milk. Continue to add, alternating and beginning and ending with flour mixture.
Toss your mini chocolate chips in about 1 tsp of flour until they are lightly dusted and gently fold them in to your cake batter (this will help keep them from sinking). Immediately spread into prepared pan.
Bake on 350F for 18-22 minutes, checking with a toothpick to ensure done-ness
Allow cake to cool completely before frosting.
In stand mixer or with electric mixer, cream butter.
Gradually add powdered sugar until combined.
Add heavy cream, about 1 Tbsp at a time, and then vanilla extract.
Increase mixer speed to high and beat for 1 minute.
Spread frosting over cooled cake.
Sprinkle with chocolate chips and then drizzle or pipe (I used a small ziploc bag to haphazardly pipe the fudge on top of the cake) hot fudge sauce over cake.