1Preheat oven to 250 degrees Fahrenheit. Using the 2 teaspoons butter, grease two 12 by 15 inch baking sheets and place them in the oven.
2In a small bowl, combine the baking soda, the 1 teaspoon of water and the vanilla and set aside. In a large saucepan, combine the 1 cup water, the corn syrup and sugar. Insert a candy thermometer into the sugar mixture and cook over medium heat, stirring to dissolve the sugar for about 4-5 minutes, or until the mixture is clear and the thermometer reads 240 degrees.
3Stir in the 3 tablespoons of butter and the peanuts and cook, stirring constantly, until the mixture has thickened and the thermometer reads 300 degrees. Be careful that it doesn't burn.
4Remove from the heat and immediately stir in the baking soda mixture. Remove the baking sheets from the oven. Pour half the candy mixture onto each warm baking sheet and quickly spread evenly about 1/4 inch thick.
5Set aside to cool for about an hour. When cool, lift the edges of the brittle and break it into pieces. To store, pack in single layers, separated by waxed or parchment paper in airtight containers. It will keep for up to one month.