1. Preheat oven to 350 degrees Fahrenheit (180 degrees Celsius).
2. In a mixing bowl, cream margarine and light brown sugar. Add eggs, one at a time, beating well after each addition. Beat in vanilla.
3. Combine the flour, baking powder, and salt; gradually add to creamed mixture. Stir in nuts.
4. Divide batter in half (there should be approximately 1 and 3/4 cups of batter in each half). Stir chocolate or cocoa and oil into one portion; spread into a 9×13-inch baking pan lined with Gefen Parchment Paper. Take spoonfuls of remaining batter and place at intervals on top of chocolate batter. With a wet spatula, gently spread second layer evenly over first layer.
5. Bake for 30 minutes or until brownie begins to pull away from sides of pan. Cool.
Brownies are best when they are underbaked, so always check them at least five minutes before the allotted time. They stay chewy and moist that way. It’s also recommended that you freeze brownies, as this keeps them moist as well.