Here is a guide for 11 miscellaneous achievements around Paradise Killer, with this one being the ninth one.
1. You have to scale the mountain that makes up a lot of the map. There is a walkway, that brings you essentially to the top of the map with the giant doors.
2. Once inside, you'll see walkways that don't line up. Line them up and proceed inside.
3. Go through the now accessible doors, walk forward, turn left, and open the gate to proceed up and out.
4. Up and to the left is a rock, and when you jump on it, is a vending machine.
5. Use it to buy the chocolate!