In a large saucepan, melt butter and add brown sugar. Bring this mixture to a boil, stirring frequently. Boil and stir for 1 minute until thickened. Remove from heat and add milk. Beat with wire whisk until smooth. Add vanilla and beat again. Then add enough powdered sugar until of desired spreading consistency; the frosting should be POURABLE. POUR frosting over cake from pan and spread to cover. Frosting cools VERY QUICKLY.