I am sensitive to Caffeine, how could I prepare a chocolate-like sauce?

There's actually very little caffeine in chocolate. So little in fact that decaffeinating it in any kind of economical fashion is next to impossible. Scientists have done it in the lab, but doing it on a scale where you could actually buy it at a reasonable price remains out of reach for most. If you feel like chocolate leaves you 'wired' it's probably down to the theobromine which is another chemical present in chocolate which is structurally very similar to caffeine, but it has a more pronounced affect on your body and heart rate than your head usually.

I'm not sure if this would work, but you could try steeping cocoa powder multiple times. Caffeine (and I'd assume theobromine as well) are very water soluble. For example, with tea approximately 75% of the caffeine will dissolve out of the tea leaves into the water after a single steeping. If you took cocoa powder and steeped it in hot water, and then filtered it through a coffee filter, and did this a few times you should be able to eliminate most of the caffeine and theobromine. You could then use the cocoa powder to make some sort of chocolate-like sauce by adding in some sweetener and coconut oil to make it vegan.

Or you could use carob.

There are products which are used as a substitute for cocoa. Carob comes to mind. While it obviously doesn't have the same aroma, you can still try making the substitution. Just use carob powder instead of cocoa powder.

If you also have to replace cocoa butter, it becomes more difficult. No other fat performs exactly like cocoa butter, but at least this is a sauce, not a chocolate bar. So shea butter can work, or, if enough liquid is included in the recipe, coconut oil. The fats alone won't taste of cocoa, you'll have to replace a "cocoa powder plus cocoa butter" or "dark chocolate" ingredient with a "shea butter plus carob powder" substitution.

While chocolate is technically very low in caffeine, it has a not-insignificant amount of theobromine, a metabolite of caffeine. Unfortunately, if you are so sensitive to caffeine that Hershey's chocolate syrup (and I would suspect decaffeinated coffee) affects you, there may not be much you can do with raw chocolate. While CO2, solvent, and Swiss Water Process decaffeination could be applied to cocoa, there don't seem to be many on the Market. A quick search brings up a product called Wondercocoa. An avenue you may wish to explore is synthetic chocolate flavoring, or natural extracts of chocolate flavor and making your own syrups with them.