Preheat oven to 350 degrees. Beat egg yolks and add half cup of milk. Stir in brown sugar, salt, cornstarch and 1.5 cups milk. Mix well and cook over medium-heat, stirring constantly until bubbly and thick. Remove from heat and stir in butter and vanilla. Pour into prepared pie shell. Beat egg whites until frothy. Gradually add granulated sugar and beat until stiff peaks form. Spread meringue over pie and bake in preheated oven until meringue is golden brown.