Preheat oven to 350°F (175°C). Grease a 9-inch square baking pan.

In a large bowl, mix together vegetable oil (or melted butter), granulated sugar, and 1 teaspoon of vanilla extract.

Beat in eggs one at a time until well blended.

In a separate bowl, whisk together all-purpose flour, 1/3 cup cocoa powder, baking powder, and salt.

Gradually stir the dry ingredients into the egg mixture until combined. Fold in chopped nuts if using.

Spread the batter evenly into the prepared baking pan.

Bake for 20-25 minutes, or until the brownies begin to pull away from the edges of the pan.

Allow the brownies to cool completely in the pan on a wire rack.

For the frosting: In a medium bowl, cream together 3 tablespoons of softened butter, 3 tablespoons of cocoa powder, corn syrup (or honey), and 1/2 teaspoon of vanilla extract.

Gradually mix in powdered sugar and milk, beating to a spreadable consistency.

Spread the frosting over the cooled brownies.

Cut into squares and serve.