Stir together brown sugar and butter in a heavy saucepan over low heat, stirring constantly, until sugar melts and mixture is blended. Stir in 1/2 cup milk until blended.
Whisk together remaining milk, flour, and salt; stir into brown sugar mixture, and cook, whisking constantly, over medium heat 5 minutes or until thickened. Gradually add eggs, and cook, whisking constantly, 2 minutes. Remove from heat, and let cool. Stir in pecans and vanilla.