Preheat oven to 350 degrees F. Grease and flour 3 (9-inch) cake pans.
Using an electric mixer, cream butter until fluffy. Add granulated sugar and continue to cream well for 6 to 8 minutes. Add eggs 1 at a time, beating well after each addition. Add flour and milk alternately to creamed mixture, beginning and ending with flour. Add vanilla and continue to beat until just mixed. Divide batter equally among prepared pans. Level batter in each pan by holding pan 3 or 4-inches above counter dropping it flat onto counter. Do this several times to release air bubbles and assure you of a more level cake. Bake for 25 minutes or until golden brown.
For the Icing:.
Cook 3/4 cup of sugar in a cast iron skillet until brown. Stir constantly. Melt 3 cups sugar in milk.
Add milk mixture to caramelized sugar Cook until it forms a soft ball. Remove from heat & add margarine.
Beat until it cools & loses gloss. Spread between cake layers and frost the cake with it fast before it starts to set up.
Layer the pecans between the layers of cake and arrange them on the top and sides of the cake.