Preheat oven to 350 degrees F. Grease and flour a 13 by 9 by 2-inch pan.
For the cake: Using an electric mixer, cream butter and sugar together until light and fluffy. Add eggs 1 at a time, beating well 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 pan and bake for 25 to 30 minutes. Just before cake is done, prepare frosting.
For the frosting: Put butter, chocolate chips, sugar and evaporated milk in a saucepan over medium heat and bring to a boil. Stirring ingredients together, boil for 2 to 3 minutes. Remove from heat and stir in vanilla and nuts. Pour over warm cake.