Heat the oven to 350 degrees. Spray a loaf pan with nonstick cooking spray, line the bottom with a strip of parchment (make it into a sling so you have handles coming out the sides), and set aside.
In a medium bowl, beat the egg whites with the kosher salt on low speed until foamy. Increase the speed to medium until soft peaks form. Add half the sugar and continue to beat until stiff peaks are formed when the beater is removed from the bowl.
Melt the chocolate and butter together in the top of a double boiler set over a pot of simmering water. When just melted, remove from the heat and stir in the remaining half of the sugar, the egg yolks, and the flour. Stir until just combined.
Mix ⅓ of the beaten egg whites into the chocolate mixture to lighten the batter. Then carefully fold the remaining whites into the batter just until the batter is smooth and no white streaks remain.
Pour the batter into the prepared loaf pan and bake for 35 minutes until the cake just feels slightly firm in the center. Cool the cake in the pan then grab the parchment sling and carefully remove the cake from the pan.
