Cook the sugar, corn syrup, and peanuts until the mixture starts to caramelize, around 325°F / 160°C.
Add the baking soda, stirring quickly. Pour onto a nonstick rubber mat and spread out with an offset spatula until it is about 1cm / .4 in thick.
After it cools, break apart into desired pieces (about 5cm / 2 in). Reserve in an airtight container with silica gel packets to prevent the brittle from absorbing moisture.