Preheat oven to 350 and line a 9"x9" pan with parchment paper and spray with cooking spray. 

cream the butter and sugar together until fluffy

stir in the cocoa powder and mix until combined, and add the eggs and the vanilla, and mix until combined.

Combine the hot fudge sauce and chocolate chips in a microwaveable bowl and microwave in 15 second increments until melted. When evenly melted, pour into the batter mixture and mix well

Add the buttermilk to a measuring cup and then add the baking soda and stir. Then add this into the batter mixture along with the flour - and mix well

Pour the batter into the lined and sprayed baking pan and bake for 45-55 minutes or until a toothpick inserted into the middle comes out clean.

While the cake is cooking make the frosting: using a large bowl and a mixer, beat the butter and peanut butter together. Add in the powdered sugar in 1/2 cup increments, mixing as you go. Add in the heavy cream one tbsp at a time until consistency is reached

When the cake is cooled, spread the frosting as you wish - enjoy!