In a mixing bowl, cream butter and sugar. Add eggs, one at a time, beating well after each addition. Combine the flour, cocoa, salt, baking powder and baking soda. Mix the milk and yogurt (in the measuring cup). Add the wet and dry mixes to creamed mixture alternately. Fold in walnuts.Pour into a greased 9-in. x 5-in. x 3-in. loaf pan. Bake at 350 degrees F for 55-60 minutes or until a toothpick inserted near the center comes out clean. Cool for 10 minutes before removing from pan to a wire rack.