Melt butter in a large saucepan. Whisk in cocoa and oil until smooth. Cook and stir over low heat until cocoa is blender. Remove from the heat; stir in sugar. Add eggs, one at a time, stirring well after each addition. 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 25-30 minutes or until a toothpick inserted near the center comes out clean. Cool. Dust with confectioners' sugar. Garnish with pecan halves if desired.