Preheat oven to 325 degrees Fahrenheit.
Spray a 9 cake pan with extra virgin olive oil cooking spray, or just grease it with a thin layer of butter.
Dust cocoa all over the inside of the pan, tapping to evenly distribute.
Cut a round of parchment paper and line the bottom of the pan, then spray the parchment lightly.
Drain and rinse beans in a strainer or colander. Shake off excess water. Place beans, 3 of the eggs, vanilla, stevia (if using) and salt into blender. Blend on high until beans are completely liquefied. No lumps! Whisk together cocoa powder, baking soda, and baking powder.
Beat butter with sweetener until light and fluffy.
Add remaining two eggs, beating for a minute after each addition.
Pour bean batter into egg mixture and mix.
Finally, stir in cocoa powder and water (if using), and beat the batter on high for one minute, until smooth. Scrape batter into pan and smooth the top.
Grip pan firmly by the edges and rap it on the counter a few times to pop any air bubbles.
Bake for 40-45 minutes. Cake is done with the top is rounded and firm to the touch.
After 10 minutes, turn out cake from pan, and flip over again on to a cooling rack. Let cool until cake reaches room temperature, then cover in plastic wrap or with cake dome (I use an overturned plastic chip bowl). For BEST flavor, let cake sit overnight. I promise this cake will not have a hint of beaniness after letting it sit for eight hours! If you are stacking this cake, level the top with a long serrated knife, shaving off layers until it is flat and even. Frost immediately before serving.