Lightly butter 15"x10"x1" pan. Put first 3 ingredients in large bowl.
In large deep heavy saucepan, combine evaporated milk and sugar. Bring to rolling boil and cook rapidly, stirring, exactly 9 minutes. Pour over ingredients in bowl and mix well. Beat with electric mixer or rotary beater only until well blended. Stir in vanilla and chopped pecans or walnuts.
Spread evenly in prepared pan. Chill 12 to 24 hrs.
Cut in 1-1/2" squares. Top with pecan halves if desired. Store in airtight containers.