Generously grease and flour bottom only of 13x9 pan.
In large bowl, combine all cake ingredients except walnuts, beat at low speed until just moistened.
Beat 3 minutes at medium speed.
Stir in walnuts or pecans. see Note* below.
Bake at 350* for 40-45 minutes or until top springs back when touched lightly in center.
Cool one hour or until completely cooled. In medium saucepan, melt margarine or butter.I do this until slightly browned,,do not burn.over low heat, for 25 minutes, stirring constantly.
Add milk, continue cooking until mixture comes to a rolling boil.
Remove from heat. Gradually add powdered sugar and vanilla. Mix well.
If needed, add a few drops of milk for desired spreading consistency. Spread over cooled cake.
NOTE*.whenever I use nuts, pecans or walnuts, ext, I usually chop according to recipe and either toast in oven or place in dry skillet over low heat and "cook" just until fragrant.being careful not to burn.I think it helps bring out the nutty flavor by helping to release the nut oils.
To make cupcakes, fill 24-30 paper lined muffin cups, 2/3 full with batter. Bake at 350* for 20-25 minutes. Frost with caramel frosting.