Made some brittle last night so I could add to this thread...wanted to make certain I could make it right.
I didn't use a candy thermometer. I don't own one. The comment about hard crack stage is correct. Has to get to 300F - 310F or it won't get hard.
I used the meat thermometer until it hit 220F then had to use the cold water test. It was spot on accurate. Once it reached 220F, I stood over the brittle and stirred it constantly, slowly raising the temperature.
Using a metal spoon, every minute or so, I would dip the spoon into the liquid brittle mixture and ever so gently drop a tiny little ball into the cold water. When it cooled, I just pulled it out of the dish. When it came out as an intricate little stringy thing that broke in my fingers, I knew I had brittle.
I had made a chocolate/cherry sauce in the blender before I started cooking the brittle. I took the mixture off the stove immediately when it reached hard brittle stage and put 3T of the chocolate/cherry sauce into the brittle and stirred it...then immediately poured it into the waiting pan and let it sit for 10 mins on the counter top.
After 10 mins, I put it in the refrigerator for 20 minutes and pulled it out, and tested the brittle. After breaking off a few small pieces, I popped it into the freezer for 30 minutes. When it came out, WOW! totally delish, totally ready to go.
It was actually ready when it came out of the fridge, but I wanted to cool it more. It was more brittle...strength-wise when I took it out of the freezer. It is PERFECT.
Not so hard. But took a while to make. Totally worth it.
Hope it worked out for you.