Melt butter in a large pan. Whisk in cocoa and oil until smooth. Cook and stir over low heat until cocoa is blended. Remove from heat and stir in sugar. Add eggs, one at a time, stirring well after each. Stir in vanilla. Combine flour, baking powder and salt; add to cocoa mixture. Stir in chocolate chips and nuts. Spread into a greased 13 x 9 x 2-inch baking pan. Bake at 350° for 25 to 30 minutes, or until toothpick inserted near center comes out clean. Cool. Dust with confectioners' sugar, if desired. Makes 2 dozen brownies.