Generously butter baking sheets.

Combine brown sugar, white sugar, water, and butter in a large saucepan over medium heat. Bring to a rapid boil; stir in pecans. Continue to cook and stir until large bubbles form on the surface, the pecans begin to look sugary, and a candy thermometer registers 240 degrees F (115 degrees C). Remove from heat and drop by rounded spoonfuls onto prepared baking sheets. Let cool completely.