- Grease a large baking tray and a wide spatula and set aside.
- Combine the sugar and corn syrup in a large microwave-safe bowl or measuring cup. Microwave on high for four minutes. Remove from the microwave, stir in the remaining butter and the peanuts, return to the microwave and heat on high for another three to five minutes. The mixture should become thick and bubbly and turn a golden pale brown color. Remove from the microwave carefully— it will be very hot. (If not using a microwave, combine the sugar, corn syrup, and peanuts in heavy saucepan and stir over a medium heat until the sugar dissolves. Increase the heat to high and boil for about 20 minutes until a candy thermometer registers 295 degrees, stirring frequently.)
- Stir in the vanilla and baking soda; the mixure will foam up. Pour immediately onto the greased baking sheet and use the spatula to flatten the mixture out as thinly as possible.
- When completely cooled and hardened, break into pieces and store in an airtight container for up to one week.