Preheat the oven to 350°F and grease a non-stick 9-inch springform cake pan with oil and dust with almond flour.

Break the chocolate into small pieces and place with butter (if using) in a heatproof bowl. Fill a pot with 1–2 inches of water and bring to a boil, then reduce the heat to a gentle simmer. Set the bowl over the pot, making sure the bottom of the bowl does not touch the water. Stir with a rubber spatula until the chocolate is fully melted, then set aside to cool.

In the bowl of a stand mixer fitted with the whisk attachment, whip the egg whites until soft peaks form. Gradually add the ¼ cup sugar and the salt, and continue whisking until stiff peaks form.

Meanwhile, in a large bowl, beat the egg yolks with the remaining sugar until light in color and fluffy.

Add in the almond flour and the melted chocolate and mix until fully incorporated.  

Add 2–3 tablespoons of the whipped egg whites to the chocolate mixture and stir to combine. Then, working in two or three batches, gently fold in the remaining whipped egg whites until just incorporated. Be careful not to overfold.

Pour the cake mixture in the prepared cake pan and bake for 25-30 minutes, or until a cake tester comes out clean. 

Let the cake cool completely, then run a thin knife around the inside edge of the cake pan before releasing the latch. Remove the ring, then slide a knife between the crust and the pan base to release and remove the cake.

Dust with powdered sugar and serve.