In a shallow 6 inch saucepan, thoroughly mix brown sugar and flour.
Stir in butter and milk, and cook over medium-high heat until mixture comes to a rolling boil.
Boil exactly 1 1/2 minutes.
Remove from heat, allow to cool 5 minutes.
Beat with a wooden spoon until mixture begins to thicken.
Mix in vanilla and continue beating until frosting is the consistency of thick honey, no more than 1 or 2 minutes.
Immediately, pour caramel frosting onto cooled cake and quickly spread with the back of a warm spoon before frosting begins to set up.
Quickly sprinkle top of cake with toasted pecans (if desired.)
Frosting will set up completely in a few hours.
Enough for 2 loaf size cakes or one 10 inch tube cake.