Preheat oven to 350 degrees Fahrenheit (180 degrees Celsius).

In a mixing bowl, cream margarine and light brown sugar. Add eggs, one at a time, beating well after each addition. Beat in vanilla.

Combine the flour, baking powder, and salt; gradually add to creamed mixture. Stir in nuts.

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.

Bake for 30 minutes or until brownie begins to pull away from sides of pan. Cool.

In a small saucepan, melt margarine and chocolate, if using. Alternately, melt margarine and remove from heat. Stir in remaining ingredients with a whisk until desired spreading consistency is reached.

Frost cooled brownies. Before frosting hardens, use a fork to make crisscross marks all over the top. Cut into squares.