Gather the ingredients including bittersweet chocolate, unsalted butter, granulated sugar, eggs, cocoa powder, salt, and optional ingredients like vanilla extract or espresso powder.
Preheat your oven to 375°F (190°C) and grease a 9-inch round cake pan. Line the bottom with parchment paper and grease the paper as well.
In a heatproof bowl set over a saucepan of simmering water (double boiler), melt the bittersweet chocolate and unsalted butter together, stirring occasionally until smooth. Alternatively, you can melt them in the microwave in short bursts, stirring between each burst.
In a separate large mixing bowl, whisk together the granulated sugar and eggs until well combined.
Gradually pour the melted chocolate mixture into the sugar and eggs mixture, whisking constantly until smooth and well combined.
Sift the cocoa powder and salt into the chocolate mixture, and gently fold them in until fully incorporated. Be careful not to overmix.
Pour the batter into the prepared cake pan and smooth the top with a spatula.
Bake the flourless chocolate cake in the preheated oven for about 25-30 minutes, or until the top is set and a toothpick inserted into the center comes out with moist crumbs. The center may still be slightly soft, but it will firm up as it cools.
Allow the cake to cool in the pan for about 10 minutes, then carefully invert it onto a wire rack to cool completely. Once cooled, dust with powdered sugar or cocoa powder if desired, and serve slices with whipped cream, berries, or ice cream.