How did it get there? What are you working with at your disposal?

Hot water & mop.

A long stick with bathroom tissue.

Lay on some sanitary spot/ground and try to rub it off. Without making a mess for someone else

Find a friendly dog. Idk never had this issue.
Good luck

I'll tell you the best fool proof way to get it off, but first you have to tell the truth about how it got there...

Lick it off, otherwise it is a waste of chocolate. You do not want to waste chocolate, you simply don't.

How did it get there? Call a land whale hooker. She'll lick it off and give you a discount.

Did you fall on your back in San Francisco?

Ask the man who put the chocolate on your back to lick it off. Problem solved.

You’re welcome.

Congratulation! Here's your prize! https://www.youtube.com/watch?v=FTzN6bwPNJw

Perhaps attach a damp cloth to a door frame and drag your back across the edge to scrape it off. Then repeat with another cloth that is more wet to more thoroughly clean the back after most of the chocolate is scraped off.

Or toss a damp cloth towel over your shoulder, hold it at the top with one hand and grab the other end down at your waist with the other, then 'floss' your back with the towel (how some dry their back after a shower).

EDIT: Oh, if you have a can of compressed air available (such as that used for blowing out computers and keyboards and such), have someone turn it upside down and spray out the freezing liquid over the chocolate (being careful to not spray the liquid onto too much skin) so that it will flash-harden the chocolate so it can just crumble and fall off as you flex your back.

If in a office use some copy paper rolling it tight, tape it off, and try to get at it that way

It's cum, isn't it?

Why is it stuck? Is your back really hairy? Sit in the sunlight so it melts, then put a paper towel on the back of your chair to rub against to wipe it off