Preheat the oven to 350°F (175˚C). Grease a 9-inch (23-cm) springform pan with butter, fit the bottom with a round of parchment paper, and grease the paper.
Combine the chocolate and butter in a large heat safe bowl, set it over a saucepan of simmering water, and stir frequently until the chocolate and butter are completely melted. Stir in hot espresso. Remove from the heat and let the mixture cool slightly. Whisk in the vanilla extract, salt, and ⅓ cup (65 g) of the sugar. Once cool, add the egg yolks one at a time, whisking well after each addition. Add flour and combine with a rubber spatula. Set aside. In a medium bowl, use an electric mixer set at medium-high speed to beat the egg whites together with a pinch of salt. Beat until soft peaks form, then add the remaining ⅓ cup (65 g) sugar a little at a time, and beat on high until you get stiff, glossy peaks.
Add about a third of the egg whites to the chocolate mixture and gently fold it in using a rubber spatula. Add the remainder of the egg whites, carefully scooping the mixture under and over itself until no streaks remain, trying not to preserve the air in the egg whites.
Pour the batter into the prepared pan. Using a rubber or offset spatula, smooth the top of the batter. (Do not tap the cake on the counter, it will deflate.)
Bake for 25 minutes for a molten center, 28 for a soft center and 30 minutes for a a fully set center. (ovens temperatures may vary) The cake’s center should be slightly jiggly, and the edges firm, slightly pulling away from the edges of the pan.
When the cake is done, let it cool for 10 minutes, then gently unmold it from the pan and let it cool to room temperature. The cake will sink slightly and crack around the edges as it cools. You can also use the back of a spoon to tap the top to create a crackly top. You can decorate the cake with sugar coated citrus slices, cranberries, kumquats or any seasonal fruits. Dust the top with powdered sugar.
Serve each piece of cake with the whipped cream or good quality vanilla ice cream