Prepare and bake your chocolate cake following instructions on the cake mix box. When the cake is done cooking, take it out of the oven and poke the cake all over with a fork.
Microwave your hot fudge and pour 3/4 of the jar on top of the cake so it will soak in.
Combine cream cheese and sweetened condensed milk in a large bowl or stand mixer and beat until smooth. Add peanut butter and continue mixing until thoroughly incorporated. Add half of the cool whip and continue mixing it until smooth.
Spread peanut butter mixture evenly over cooled cake. Cover and refrigerate until cool.
Spread remaining cool whip over peanut butter mixture. Drizzle rest of hot fudge over top. Then sprinkle the crumbled reese's peanut butter cups over top. Store in the fridge until ready to serve.
Notes
Depending on your fudge you may have to use something a little bigger than a fork for the holes to get the fudge to seep in a little more or move the fork around while in the cake.