Prepare your pan: Heat your oven to 350°F (175°C). Line the bottom of a 6-inch springform pan with 2 1/2-to-3-inch sides with parchment paper and coat the sides lightly with butter or nonstick spray. If you’re worried the springform may be leaky, wrap the outside of the pan tightly with a piece of foil. Place on a baking sheet.

Make the batter: Melt the chocolate and butter together in a large bowl in a microwave in 30-second bursts, stirring between each, or with the bowl set above a pot of barely simmering water on the stove. Remove from heat and whisk in half the sugar (1/4 cup or 50 grams), the salt, and the cognac. Whisk in the yolks one at a time until smooth. In a medium-large bowl, beat the egg whites with an electric mixer until thickened like loose whipped cream then gradually add remaining 1/4 cup (50 grams) sugar, beating the whole time, until soft peaks form. Spoon about 1/4 of the egg whites into the chocolate mixture and whisk them in; this lightens the batter. Add remaining egg whites to chocolate mixture, gently folding them in until there are no egg white streaks remaining.

Bake the cake: Pour into prepared pan and bake for 20 to 25 minutes or until the center is domed, only moves slightly when the pan is wiggled, and a toothpick inserted into the center comes out batter-free (some gooey crumbs are fine). Transfer to a cooling rack and while you can cool it completely in the pan, I’m convinced you get better flakes on top if you loosen the springform sides when it’s halfway cooled. First run a knife around the sides to loosen the cake anywhere it’s stuck. The cake deflates as it cools and the center will sink slightly.

To serve and/or make ahead: I prefer this cake fully cold so I transfer it to the fridge to cool the rest of the way, about an hour or two. Dust with powdered sugar and serve in small wedges with cream and berries, if you wish. Cake keeps for a week in the fridge, although this is inconceivable to us, and longer in the freezer.