Grease a quarter sheet pan and lay an over sized piece of parchment paper onto it. Leave a little lip to help remove the brittle later on.
In a large stainless steel pot, pour sugar. Pour water on top. Bring mixture to low heat, stir constantly. Once the sugar has dissolved, increase to medium heat.
Place candy thermometer in the pan. When the sugar reaches 280F, dump in butter. Stir.
When the temperature reaches 300F, stir in peanuts. Continuously stir until mixture reaches 310F. Immediately remove from heat, stir in baking soda and pour onto lined and greased pan, carefully. This mixture is extremely hot, and sugar likes to stick to what it lands on.
Once sugar has cooled completely (this will take around 2 hours), use hands to break up the pieces and enjoy!