Preheat oven to 350 degrees F. Grease and flour a 13 x 9 by 2" pan. For the cake: Using an electric mixer, cream the butter and sugar together until fluffy. Add eggs, one at a time, beating after each addition. Sift flour, baking powder, and salt together in another bowl. Add to creamed mixture, continuing to beat. Add vanilla and chocolate syrup to batter and mix well. Pour into prepared panand bake for 25-30 minute Just before cake is done, prepare frosting. For the frosting: Put butter, chocolate syrup and evaporated milk in a saucepan over med. heat and bring to a boil. Stirring ingredients together, boil for 2 to 3 minute Remove from heat and stir in vanilla and nuts. Pour over warm cake.