Combine the brown sugar and evaporated milk in a medium saucepan. Heat over medium-high heat, stirring to dissolve the sugar. When the mixture begins to boil, connect a candy thermometer to the side of the pan.
Heat to 232 degrees F (112 degrees C), or until a small amount of syrup dropped into cold water forms a soft ball that flattens when removed from the water and placed on a flat surface.
Remove from the heat and use a sturdy wooden spoon to stir in vanilla. Add butter and pecans and continue to stir until thick and creamy. It will lose some of its shine. Drop spoonfuls onto waxed paper and allow to cool at room temperature until set. Store in an airtight container at room temperature.