Crank the oven to 300°F.
Grease an 8-inch square baking pan with nonstick oil spray. Trim a piece of parchment paper so that it fits just inside the baking pan with a 2-inch overhang on two sides and place it inside the pan. This is a sling to allow for easy removal.
Sift together the sugars, cocoa powder, flour, and salt into a medium bowl.
Using a stand mixerYou can also use an electric hand mixer if that’s what you’ve got. fitted with the paddle attachment, beat the eggs on medium speed until fluffy and light yellow, 2 to 3 minutes. Reduce the mixer speed to low and slowly introduce the sugar mixture, followed by the butter and vanilla. Mix just until smooth and scoop into the prepared pan. (The batter will be very thick...that's okay.)
Bake for 15 minutes, then remove from the oven and let cool for 15 minutes. Then return the pan to the oven and bake until an instant-read thermometer inserted into the middle of the brownie reads 195°F, approximately 30 minutes.
Cool in the pan for 30 minutes, then transfer the brownie to a cooling rack for 10 minutes before cutting into 16 pieces using a pizza wheel or long slicing knife.
I'd tell you how to store them by stacking them in an airtight container, separating the layers with wax paper, but you know good and well that isn't going to happen? Don't you?