Preheat oven to 350 degrees.
Cream butter in mixing bowl with electric mixer for 1 minute.
Add sugar gradually, then eggs and vanilla, beating until well combined, about 2 minutes.
Scrape down sides of bowl, if necessary.
Set aside.
Whisk together flour, cocoa, baking powder, and baking soda.
Add dry ingredients alternately with sour cream into creamed mixture until well combined.
Fold in walnuts and chocolate chips.
Spread evenly into a buttered and floured 13 x 9 inch baking pan.
Bake 35 minutes if using a metal pan, 25 minutes if using a glass pan.
Test at with a toothpick.
Do not over bake.
Cool thoroughly before frosting with Sour Cream Chocolate Frosting.