Before starting, calibrate your candy thermometer: Bring water to a boil and take the temperature with your thermometer. It should read 100°C (212°F). If not, take note of the difference during cooking.
In a small bowl, mix the bicarbonate, water, and vanilla. Set aside.
Mix the sugar, water, and corn syrup in a large saucepan.
Cook over medium heat, stirring occasionally, until the candy thermometer reads 240°F (115.5°C) or until a small amount of syrup, dropped into very cold water, forms a soft ball that flattens when removed from the water.
Stir in the butter and peanuts.
Cook, stirring constantly, until the mixture reaches 300°F (150°C) or until a small amount of the mixture, dropped into very cold water, comes out hard and brittle.
Remove from heat immediately.
Quickly stir in the bicarbonate mixture and whip until light and foamy.
Pour half of the mixture onto each cookie sheet.
Spread the mixture quickly by tilting the sheets in your hands until the mixture is about 1/8 inch (5 mm) thick.
Let cool to room temperature for at least 1 hour before breaking into pieces.