I've experimented with Peanut Brittle since I was a kid. Confectionary cooking was my entry into the food world. Not only did I enjoy eating the fruits of my labor, but I've always been entrigued by the chemistry of cooking- the way temperature affects sugar particularly interests me. If you cook the sugars to too high of a temperature, your peanut brittle will not be brittle, but rock hard. If you don't bring it to a high enough temperature, it'll be peanut chew. Then, when the brittle's off the heat, you can add some baking soda and it becomes foamy instead of glossy and dense.
The keys to great peanut brittle in my opinion are: make it foamy, cook it to exact temperatures (use a good candy thermometer), and use lots of peanuts chopped up to just the right sizes -some chopped fine, some left in halves. I find an easy way to do this is pulse them in a food processor. The resulting inconsistent chop is just right. The smaller pieces blend into the sugars and flavor the candy with rich peanut taste, while the large pieces make for a nice chunky brittle.
I think the absolute best recipe is the one for "Old Fashioned Peanut Brittle" on Epicurios.com. It can be made weeks ahead of time, then stored in an airtight container so it was the first sweet I made for the holiday season. Although, I mixed up a batch a week ago and I'm almost out-with two weeks to go until Christmas. I plan on making another this week to make sure everyone gets a taste and to try to stretch it until the 25th.