Preheat oven to 350°. In a large bowl, cream butter and sugar until light and fluffy, 5-7 minutes. Add eggs and vanilla; mix well. Combine the flour, cocoa, baking powder and salt; gradually add to creamed mixture just until blended (do not overmix). , Pour batter into 2 greased 9-in. square baking pans. Sprinkle with chips and nuts. Bake until a toothpick inserted in the center comes out clean, 30-35 minutes. Cool.