1/2 cup (1 stick) butter, room temperature, plus butter for pan
1 cup sugar
2 large eggs, room temperature
1 teaspoon vanilla extract
2 cups flour
1/2 cup unsweetened cocoa powder
1 teaspoon baking powder
1 teaspoon baking soda
1 cup sour cream
1 cup chopped walnuts
3/4 cup semisweet chocolate chips
1/4 cup butter, softened
3 squares (3 ounces) unsweetened chocolate, melted
2 cups powdered sugar
1/2 cup sour cream
2 teaspoons vanilla extract
Cream butter in mixing bowl with electric mixer for 1 minute. Add sugar gradually, then eggs and extract, 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 alternating 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 x 2 inch baking pan. Bake at 350° F for 35 minutes if using a metal pan. If using glass, test at 25 minutes with wooden pick. (Do not over bake.) Cool thoroughly before frosting.
Frosting: Combine all ingredients in mixing bowl with electric mixer until smooth.