Preheat the oven to 350°F. Grease a 5 ¾ by 3 inch (2 cup capacity) loaf pan and line the bottom with a strip of parchment paper. If using a different size or small pan, just make sure the batter has room to climb and doesn't go all the way to the top of the pan.

In a microwave-safe bowl, combine the chopped chocolate and the butter and heat at 50% power, stirring every 40 seconds or so, until chocolate is melted and mixture is smooth. If you prefer, you can do this in a double-boiler or bowl set over water. The microwave worked fine.

Stir half of the sugar into the warm melted chocolate mixture, then stir in the egg yolks, and flour.

Using a handheld electric mixer, whip the egg whites with the salt until foamy. Gradually add remaining sugar and whip until whites hold stiff peaks but are not dry.

With a rubber spatula, fold one-third of the egg whites into the chocolate mixture to lighten, then fold in the remaining egg whites just until mixture is smooth and whites are fully blended.

Scrape the batter into the prepared loaf pan, smooth the top, and bake for 30 minutes, just until the cake feels slightly firm in the center. Do not overbake.

Let the cake cool in the pan before serving.