Preheat the oven to 350°F (180°C).
If you are going to use the fan, 320°F (160°C) should be enough.
Line bottoms of two 8" or 9" springform pans with parchment paper.
Using a stand electric mixer, beat 9 eggs with the whisk attachment for 1 min on high spped.
With the mixer on, gradually add 1 cup erythritol and continue beating on high speed a full 8 min.
It will be thick and fluffy.
Tip: You will need a huge mixing bowl.
Whisk together almond flour and cocoa powder and sift in thirds into batter.
Fold with a spatula between each addition.
Once all flour is in, continue to fold just until no streaks of flour remain, scraping the bottom of the bowl to get any pockets of flour.
We do not want to over-mix and deflate the batter.
Gently fold in ½ Tsp vanilla and room temp butter, folding as you add butter in a steady stream and scraping from the bottom to make sure you don't have butter pooling at the bottom.
Fold just until incorporated.
Pour 1/3 and 2/3 of batter into the prepared pans (the 2/3 is going to be split in two when baked).
Bake immediatelyfor 20 minutes, or until a toothpick comes out clean.
If the batter is not baked straight away after mixing, it might deflate.
Let cool in pans for 10 min then run a thin spatula around edges to loosen cake.
Transfer to a wire rack and remove parchment paper.
When cakes are at room temp, take the thicker one and devide it in half lenghtwise using a cake slicer or a long sharp knife.