Preheat oven to 350°F. Lightly grease the sides of an 8-inch round non-stick springform pan with butter and line the base with a round of parchment paper.
Coarsely chop the chocolate and place it in a saucepan with the butter over very low heat. Stir constantly until completely melted and smooth, being sure not to burn it. Set aside to cool.
Place eggs in the bowl of a stand mixer fitted with the whisk attachment and beat for 1 minute on high speed until very foamy. Gradually add sugar one tablespoon at a time and continue to beat for 6 minutes longer on medium-high speed until very pale, thick and voluminous. Slowly dribble in the melted chocolate mixture bit by bit while folding gently by hand with a wide spatula. Pause to sift in the cocoa powder and then continue folding gently until evenly combined. Reach down to the bottom of the bowl to lift up the dense chocolate mixture that can sink to the bottom. Try not to over-mix as it will knock the air out of the batter.
Pour batter into a prepared pan and then place the pan onto a rimmed baking tray. Bake for about 30 minutes until the cake is puffed and slightly cracked on the surface. It will look dry and feel a bit firm, but trust that it will be tender. Transfer cake pan out onto a wire rack to cool completely.
Run a knife around the edges of the pan and gently release the sides. Dust with cocoa and serve with whipped cream if desired. This cake is best served at room temperature. Slice with a hot dry knife.