Preheat the oven to 350°F and spray a 9" cake pan with cooking spray. If using a springform pan, line the entire pan with a piece of parchment paper.
In a large mixing bowl, stir together the dry ingredients (flour through salt). Add the wet ingredients (coconut oil through water) and stir just until combined. The batter will be extremely thin.
Pour into the prepared pan and bake for 27-32 minutes. The toothpick test doesn't work here. You'll need to check with a knife to ensure that the cake is fully cooked.
Let cool for at least 30 minutes before serving. Dust with powdered sugar before serving, if desired. The flavor and texture of this cake is best 4 or more hours after baking. Cover and store at room temperature for up to 2 days.
Notes
If you use unrefined rather than refined coconut oil, this cake may have some coconut taste to it.