You can just cook sugar and water to make a caramel syrup, and also add an acid to help invert the sugar. You can cook it to various temperatures to make a hard candi or a syrup.
Well it turns out that true Belgian candi syrup (or sugar) is based on a maillard reaction. This process needs the presence of nitrogen for the reaction and we can use DAP (Di-Ammonium Phosphate) yeast nutrient for that. When the DAP breaks down at a certain temperature we are left with acid + nitrogen. The acid also helps with the inversion process. I had some DAP from a few years ago when I bought some to add to a hard cider so I was in luck. I think they carry it at Noble Grape.
I found an old thread on HBT and read for a couple of hours to get a decent understanding before starting the process. I was originally going to make an amber syrup but things were going great with no burning at all so I took it all the way and made a nice dark syrup. The HBT thread is here if you want to read it... it is long and I went through it all. The recipe is in the first post (light, amber, dark, etc). The OP got much of the information in the book 'Brew Like a Monk' which I have not read.
Some notes and amendments:
- Open the window and get the fan going, there's lots of ammonia evaporating in the beginning.
- Take it slow. It's not worth burning it and wasting your efforts. You will be rewarded. I use a glass top range and had the dial between 2 and 3 most of the time. There's a stall around 240F just be patient.
- Don't keep stirring it. If you do you may get crystallization which you don't want. I only stirred in the beginning and when doing the water additions.
- When adding the water to cool just ad a bit at a time and stir. This will prevent crystallization.
- After the final water addition bring it back up to 240F and pour into pre-heated mason jar(s).
- It tastes awesome! Strong plum and raisin notes. Don't burn your tongue, I came close a couple of times.
I did a dark syrup but kept it under 290F by a couple of degrees. I did one pass up to 287F or so, cooled with 1 cup of water, then back up to 270F, cooled again with only about 200ml water, then back up to 240F (past the stall). That's when I poured it into heated jars (oven). The whole process took me 1.5 - 2 hours. I didn't set out to make a dark syrup but things went great so I pushed it a bit. I tasted it at 10 degree intervals and loved the flavour development.
And now for some Belgian candi syrup porn:

Ready to Make Belgian Candi Syrup by Hot Pepper Daddy, on Flickr

Candi Syrup Starting to Boil by Hot Pepper Daddy, on Flickr

Candi Syrup Turning Rose Colour by Hot Pepper Daddy, on Flickr

Candi Syrup by Hot Pepper Daddy, on Flickr

Candi Syrup Almost Done by Hot Pepper Daddy, on Flickr

Nice Colour by Hot Pepper Daddy, on Flickr

Candi Syrup Rainbow by Hot Pepper Daddy, on Flickr

Finished Dark Belgian Candi Syrup by Hot Pepper Daddy, on Flickr
I'm going to brew with the large jar and save the small one so people can taste it at the meeting this week.
If anyone tries this let me know and we can also compare!
-Rob