Add the flour and salt to the mixing bowl and fold it into the wet ingredients by hand using a rubber spatula. Be sure to do this carefully so you don't deflate the batter as the flour is incorporated. Folding requires that you scrape the bottom of the bowl and use an "up and over" motion. Do not stir, as this will break the air bubbles you've created by beating the ingredients up until this point. This cake contains no leavening agent, so the bubbles in the batter are what will keep it from sinking in the middle as it bakes.