In a heavy 2-quart saucepan melt the butter or margarine.
Add the brown sugar.
Cook over low heat for 2 minutes, stirring constantly.
Add the evaporated milk and continue cooking and stirring till the mixture comes to boiling.
Remove from heat.
Cool till lukewarm (110°F).
Stir in vanilla.
Gradually add the powdered sugar, beating vigorously till the mixture is a fudgelike consistency (it should take for about 2 minutes).
Stir in nuts.
Spread the candy into a buttered 8x8x2-inch pan.
Cut into squares.