Preheat the oven to 350F (180C). Lightly grease a 9x13 inch (23x33 cm) pan with nonstick cooking spray, then line with baking paper so that there's an overhang around the edges. Greasing the pan first makes the paper stick to the pan and then it's easier to spread out the dough. Alternatively, simply grease the pan.
In a medium bowl, sift together the flour, cocoa, baking soda and salt. Whisk together.
In a separate large bowl, beat together the butter, granulated sugar and brown sugar until creamed.
Beat the eggs, vanilla and oil into the butter mixture.
Mix the dry ingredients into the butter mixture. I typically do this about 1/2 at a time and start with the mixer on a low speed so that the flour doesn't make a mess when you turn on the electric mixer.
Spoon/dollop the dough into the prepared pan. Spread into an even layer using an offset spatula. Alternatively, take a large piece of wax paper and press the dough down into an even layer (using the wax paper between your hands and the dough.
Bake in the preheated oven for about 24-29 minutes, or until the top looks set. The bake time is slightly subjective depending on if you prefer chewier or cake-ier bars. A longer bake time will lead to cake-ier bars.