Use a mid-range bittersweet or semisweet chocolate for this recipe. I use one with 70% or fewer cacao solids; extra-bittersweet chocolates above that percentage can lead to dry cakes.
You can either almond flour (or another nut flour) or regular flour in this recipe. You'll want to bake the cake a little longer if using almond flour since the ground nuts don't absorb moisture as readily as wheat flour. It's usually best to err on the side of underbaked with chocolate cakes like this, as they'll continue to cook when removed from the oven. Someone pointed out the crack in the top of the cake when I posted a picture online while testing the recipe. Cakes made with beaten egg whites will rise a bit and form a crust on top during baking, but will sigh and settle just a bit, and crackle a little across the top.
C'est normale.
Lastly, springform pans vary in size. If yours isn't exactly 9 inches/23cm, you can use a slightly larger one. If so, the cake will probably take a few minutes less to bake.
For more information on ingredients, check out my posts on
almond flour,
chocolate FAQs, and
cocoa powder FAQs.