Grease a baking sheet with butter and set aside. In a small bowl, mix the baking soda and salt together and set aside. In a large saucepan, combine corn syrup and sugar. Cook mixture until it reads 255 degrees on a candy thermometer. Add peanuts and stir. Stir in baking soda and salt. Be careful! Mixture will bubble up and is really hot!) When mixture starts to settle, stir in vanilla and butter. Pour mixture onto the prepared baking sheet and spread with a greased metal spatula. Let cool to room temperature. Crack the brittle into irregular chunks. Store in an airtight container at room temperature.
If using for sundaes, reserve some chunks of brittle for garnish. Grind remaining brittle into coarse peanut praline by using the pulse button on your food processor. You don't need to grind it very fine; some small chunks are good. Alternate layers of vanilla ice cream, hot fudge and peanut praline. Garnish with chunks of peanut brittle.