First, grease your jelly roll pan.
Then you can start with your sugar, corn syrup and water in a large, heavy pot, after you've sprayed the sides of the pot with cooking spray. (You don't want any flimsy pots here, it'll burn and ruin a flimsy pot. You'll be working with really high heat for an extended period of time, so seriously, use, or borrow, a nice pot.)
You also need a candy thermometer that attaches to the side of your pot and remains there the entire time.
Turn the heat on medium and stir occasionally (it should be boiling) until it reaches the thread stage, or 230' (this probably took about 5 minutes).
Next, you're going to add in your peanuts, and cook, stirring occasionally, until 301', or the hard crack stage. This took quite a while for me, about 20 minutes. It should be boiling the entire time
Brittle should be a nice, dark, amber color
Remove the pot from the heat, and this is where your boyscout mantra comes in handy (be prepared!).
Throw in the butter and vanilla, stir violently until just incorporated. But be careful because it will bubble up and steam.
Last, add in the baking soda and salt, mix it up again, until just incorporated (no longer)
Pour it immediately into your greased jelly roll pan. Slowly lift each side of the pan up at angles to get the brittle to spread out and cover the entire thing.
Let it sit <g class="gr_ gr_270 gr-alert gr_gramm gr_disable_anim_appear undefined Grammar only-del replaceWithoutSep" id="270" data-gr-id="270">for </g>about 2 hours until the bottom of the pan is completely cool.