Combine sweetened condensed milk and caramel topping, set aside.
Mix cake according package directions.
Coat the inside of a 6 cup bundt pan with non-stick spray
Pour in the cake batter until 3/4 full. You may have leftover batter.
Pour 1 cup of water in the Instant Pot and insert the trivet. Carefully lower the bundt pan on to the trivet.
Secure the lid, making sure the vent is closed.
Using the display panel select the MANUAL or Pressure Cook function*. Use the +/- buttons and program the Instant Pot for 28 minutes.
When the time is up, quick-release the remaining pressure. Carefully remove bundt pan from pot and set on a cooling rack.
While the cake is still hot, make 1 1/2 inch deep holes across the top of the cake with the handle of a wooden spoon or a straw. Pour 6 ounces of the caramel/milk topping over the cake.
When cake is completely cool, turn out onto a cake plate, drizzle with remaining caramel/milk topping and sprinkle with crushed Butterfinger.
Garnish with whipped cream and additional Butterfinger as desired.