Preheat oven to 350°F. Grease and flour a 9-inch baking pan.
In a large bowl, whisk together oil, sugar, vanilla, and eggs. In a separate large bowl, sift together flour, cocoa, baking powder, and salt. Pour egg mixture into flour mixture; blend until just mixed. Stir in nuts if using.
Pour batter into prepared baking pan. Bake 20 to 23 minutes, until the edges begin to pull away from the pan. Cool, slice, and serve.