Prepare a large, rimmed, baking sheet (or two small, rimmed, baking sheets) by coating it with butter. (I used a large, 13.5-inch x 17-inch baking sheet with a smaller, baking sheet on stand by if I decided to keep my peanut brittle thin so I could pour the remaining amount in, in case I felt my brittle was thick enough.)
Boil the corn syrup, sugar and water in a saucepan until it reaches 230° F with a candy thermometer or until it *spins a thread.
Add peanuts and stir frequently for about 15 minutes or until syrup has turned a light brown.
Remove from the burner and add in the margarine, vanilla and baking soda. (The mixture will foam, and that's ok!)
Stir quickly, and then pour out on to prepared baking sheet(s) and spread out evenly. (It will start to harden and dry quickly, so it's best to work fast.)
Allow it to cool. When cool, it should be hardened, and you should be able to pick it up and start breaking in to pieces.
Video
Notes
*A boiling a "thread" means when a candy mixture reaches a consistency that when a small amount of syrup is added to cold water, it forms a "thread" and it doesn't break.