A quick tip here, spray your measuring cup with some non-stick spray before pouring your corn syrup. This will cause the corn syrup to come out with much less problems!!
Attach your candy thermometer to the side of your pot, making sure that the bottom of the thermometer isn't sitting on the bottom of the pan. This will give you a false reading, and you may burst your thermometer! So, that being said, over medium heat, heat this mixture until your thermometer reads 245* (also known as the soft ball stage). While you're waiting, pour out your peanuts - I ended up using about 2 1/2 Cs, because I like my peanut brittle extra peanut-y.
Also, get out a baking sheet, and spray fairly generously with some non-stick spray. You don't want it to be dripping with non-stick spray, but you want to be sure it is coated evenly. Once your sugar/water/corn syrup mixture has reached 245*, stir in the peanuts, and continue cooking this mixture until you reach the 300* mark - roughly another 10-15 minutes.
Once your temperature has increased to 300*, add in the baking soda, and stir, carefully, for another couple of minutes, until your mixture has reached the color of your liking. However, remember that the caramel will continue to darken after you remove it from the heat.
As soon as your caramel is to your liking, pour it directly onto the greased baking sheet, and spread it out to an even layer. Set this aside to cool, I left mine on the counter for a couple of hours. My daughter was beside herself to have some!
Now comes the fun part! You can either whack it with a kitchen mallet, or just pick up a corner of the candy and let it drop back to the pan, thus breaking the candy. This candy is full of flavor, but not too hard to bite into. This is some seriously yummy candy, and makes a great gift! No one has to know how easy it was to make!