Soften the butter at room temperature at least 4 hours before or few seconds in the microwave. The butter must be like a paste.
Grease and line a 23 cm or 9 inches a cake springform tin with baking paper.
Preheat the oven at 180° C
Place the chocolate in a metal or glass bowl set over a saucepan of simmering water and let it melt.
When melted, remove the bowl from the heat let it cool down.
Meanwhile, separate the egg yolks from the whites and set the egg whites aside.
Place the softened butter in a stand mixer bowl ( you can also use a handheld mixer if you prefer), add half of the sugar and beat with the paddle attachment for a few minutes until the mixture is light and fluffy.
Add the egg yolks to the mixture one at a time and beat until they are well incorporated.
Add the chocolate to the mixture, and mix well.
Then add the almond flour and stir to combine the ingredients with a rubber spatula or a wooden spoon.
Whip egg whites until foamy, then gradually add the rest of sugar and whip until stiff peaks form.
Gently fold the whipped whites into chocolate mixture.
Pour batter into the prepared cake tin.
Level the top with a rubber spatula or with a spoon.
Bake at 180° C for 40 minutes.
I recommend doing the toothpick test: which must come out clean and slightly damp.
When it's cooked, let the cake cool completely in the tin over a cooling rack.
When the cake is cold, invert the cake onto a serving platter and dust with icing sugar.