Preheat the oven to 350°F (180°C). If you are going to use the fan, 320°F (160°C) should be enough. Line the 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 speed. 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, cocoa powder, salt, and cinnamon, and sift in thirds into the 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 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 ⅓ and ⅔ of batter into the prepared pans (the ⅔ is going to be split in two when baked). Bake immediately for 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 divide it in half lengthwise using a cake slicer or a long sharp knife.