Prepare the chocolate cake according to package instructions in a 9x13-inch pan. Let it cool for about 10 minutes.
Using the handle of a wooden spoon, poke holes all over the cake.
Mix the peanut butter with the sweetened condensed milk and pour over the cake, letting it sink into the holes.
Warm the hot fudge slightly and drizzle it evenly over the cake.
Let the cake cool completely, then spread the whipped topping on top.
Sprinkle with chopped Reese’s and refrigerate for at least 1 hour before serving.