Generously grease a large slab or baking sheet.

Combine brown sugar, white sugar, and milk in a saucepan over medium heat; bring to a boil. Stir in pecans, butter, and vanilla; heat, without stirring, to between 234 and 240 degrees F (112 to 116 degrees C) or until a small amount of syrup dropped into cold water forms a soft ball that flattens when removed from water and placed on a flat surface. Remove saucepan from heat; cool syrup 5 minutes.

Beat until thickened, then immediately pour onto the prepared surface; rest until firm. Completely cool before cutting.