Preheat oven to 350 degrees F (175 degrees C). Grease a 9x9 inch baking pan.
In a medium bowl, mix together the oil, sugar, and vanilla. Beat in eggs. Combine flour, cocoa, baking powder, and salt; gradually stir into the egg mixture until well blended. Stir in walnuts, if desired. Spread the batter evenly into the prepared pan.
Bake for 20 to 25 minutes, or until the brownie begins to pull away from edges of pan. Let cool on a wire rack before cutting into squares.
1/2 cup butter, softened
1/2 cup creamy peanut butter
2 cups powdered sugar
2 teaspoons milk
1 cup semisweet chocolate chips
1/4 cup butter
1. Heat oven to 350°F. Grease bottom only of 13x9-inch pan with cooking spray or shortening. (For easier cutting, line pan with foil, then grease foil on bottom only of pan.)
2. Spread in pan. Bake 28 to 30 minutes or until toothpick inserted 2 inches from side of pan comes out almost clean. Cool completely, about 1 hour.
3. In medium bowl, beat filling ingredients with electric mixer on medium speed until smooth. Spread mixture evenly over brownie base.
4. In small microwavable bowl, microwave topping ingredients uncovered on High 30 to 60 seconds; stir until smooth. Cool 10 minutes; spread over filling. Refrigerate about 30 minutes or until set. For brownies, cut into 9 rows by 4 rows. Store covered in refrigerator