Nicola Lamb's 8 Inch Flourless Chocolate Cake

Name: Nicola Lamb's 8 Inch Flourless Chocolate Cake
Prep time: 20 min
Cook time: 1 h
Total time: 1 h 20 min
Cuisine: European, Mediterranean, French
Ingredients: 170g dark chocolate (I use 70%); 110g butter (I use salted); 2 tbsp cold water; ¼ tsp sea salt flakes; 10g cocoa powder (optional, plus more to dust); 180g egg whites (around 5-6 eggs, probably 5 if large); 140g caster sugar; 35g egg yolks (around 2); a tall 8 inch cake tin (rather than a sandwich tin. Mine is loose bottomed); an electric mixer (mine is handheld)