Preheat oven to 180 degrees Celsius (350 F) and line a rimmed baking tray with parchment paper and lightly grease the parchment paper**.
Puree the zucchini in a food processor (skin, seeds and all), set aside.
In a large bowl, mix the zucchini puree, 2 eggs, vanilla extract and the chocolate. The chocolate will cool right away and become a bit grainy. That's all right, the tiny bits will melt while baking.
Add yogurt and mix well.
In another bowl, mix oat flour, baking soda, sugar and cocoa powder.
Add dry mixture to wet mixture and mix well.
Beat the egg whites with the salt until stiff peaks are formed.
Gently fold egg whites into batter until fully incorporated.
Pour batter into baking tray*** and bake for ca. 20/25 minutes.
Let cool on tray for 10 minutes before transferring to a wire rack to cool completely.
For the chocolate whipped cream, put all ingredients in a bowl and whip until it reaches desired consistency. Add sweetener to taste.
Frost cooled cake as desired****.
* Make your own oat flour: put old fashioned oats into a food processor or coffee grinder and pulse until you get a fine flour.
** As there is no added oil or butter in the batter, the cake will stick to the parchment paper.
*** You can use smaller baking pans, but you might need to adjust baking time accordingly.
**** I cut the cake into three equal parts and made a small layer cake. On the top, I added some fresh raspberries and blueberries and topped with a little bit melted (dark, sugar free) chocolate.