Gather ingredients. Preheat the oven to 350 degrees F (175 degrees C). Grease a 9-inch square baking pan.

Mix sugar, oil, and vanilla together in a medium bowl; beat in eggs until well combined.

Mix flour, cocoa, baking powder, and salt together in a separate bowl; gradually stir into egg mixture until well blended. Stir in walnuts.

Spread the batter evenly into the prepared pan.

Bake in the preheated oven until the brownie begins to pull away from the edges of the pan, about 20 to 25 minutes.

Let cool on a wire rack before cutting into 16 squares.