Preheat the oven to 350 degrees F. Line a 9" cake pan with parchment paper (set the cake pan on top of the paper and trace a circle to cut out and place on the bottom). Grease the sides of the pan with butter.
In a small pot over low heat, melt the chocolate and butter together, stirring occasionally until smooth. Mix in the sugar, vanilla, and salt, and stir until the sugar is dissolved. Turn off heat and cool slightly. Whisk in the egg yolks and set aside.
In a large bowl fitted with an electric whisk, whip the egg whites until they are light and foamy. Use a spatula to fold in the chocolate mixture until well combined.
Pour the batter into the prepared cake pan and bake until the cake puffs up and is no longer jiggly in the center, about 28 to 30 minutes. Let cool. Release the edges by running a thin knife between the cake and the pan, then carefully flip it over onto a parchment-lined board or counter. Cut with rounds and decorate, if desired. It can also be cut into triangles and eaten with a fork.