This is my favorite Peanut Brittle recipe.
3 cups sugar
2 cups water
3/4 cup light corn syrup
3/4 cup dark corn syrup
4 cups coarsely chopped salted roasted peanuts
2 tablespoons (1/4 stick) unsalted butter
1 tablespoon baking soda
1 teaspoon vanilla extract
Butter 2 heavy large baking sheets. Stir first 4 ingredients in heavy large saucepan over medium heat until sugar dissolves. Increase heat to high and boil without stirring until candy thermometer registers 260°F., about 40 minutes. Reduce heat to medium-low. Mix in peanuts and butter and cook until thermometer registers 295°F., stirring constantly, about 15 minutes. Add baking soda and vanilla and stir briskly (mixture will foam up). Immediately pour out onto prepared baking sheets, dividing evenly. Spread out brittle as thinly as possible. Let stand until cold and hard.
Break brittle into pieces. Store in airtight containers at room temperature. (Can be prepared 1 month ahead.)