In a shallow, heavy duty saucepot, combine cream and brown sugar. Insert a candy thermometer and cook over a medium heat until the mixture reaches 236 degrees. Remove from heat and stir in the pecans. Allow mixture to cool to 110 degrees, stirring occasionally. When mixture has cooled, whip in the butter and continue to whip until the mixture appears light and creamy. Drop by spoonfuls onto wax paper and allow pralines to cool completely before serving.