Preheat the oven to 350 F. Use a 9-inch cake pan with a removable bottom, or a 9-inch springform pan. Line the pan with parchment paper and spray it with nonstick cooking spray.
Place the chocolate and butter together in a large microwave-safe bowl, and microwave until melted, stirring after every 30 seconds to prevent overheating. Stir until melted and smooth. Set aside to cool to lukewarm.
Combine the egg yolks and 1/4 cup granulated sugar in the bowl of a large mixer fitted with a whisk attachment. (Alternately, you can use a mixing bowl and a hand mixer with whisk attachments.) Whip the yolks and sugar until they’re thick, a light yellow color, and drip from the whisk in a thick ribbon when you pull it up out of the yolks.
Fold the yolks into the lukewarm chocolate, and gently stir until most of the streaks are gone. Add the cranberry sauce, flour, vanilla extract, and salt, and carefully fold them in.
Wash and dry the mixing bowl and whisk you used for the yolks, and place the egg whites in the bowl. Whip on medium-high speed until soft peaks form, then gradually add the remaining 1/4 cup granulated sugar, a spoonful at a time, until the whites hold firm peaks and have the texture of shaving cream.
Fold the whites into the chocolate in three batches. Pour the cake batter into the prepared pan and bake for 32-35 minutes, until puffed, dry on top, and there are small cracks along the sides.
Remove from the oven and let cool completely at room temperature. The cake can be made several days in advance and kept, well-wrapped, in the refrigerator until ready to assemble.