Preheat your oven to 350°F (175°C). Prepare an 8-inch round cake pan by greasing it lightly.

In a heatproof bowl, combine the dark chocolate and unsalted butter. Melt gently either over a double boiler or in the microwave in short bursts, stirring until smooth. Let it cool for a few minutes.

In another bowl, whisk together the granulated sugar and eggs until the mixture is pale and slightly thickened (about 3-4 minutes).

Gradually fold the melted chocolate mixture into the egg mixture, being careful not to deflate it.

Sift in the all-purpose flour and gently fold it into the batter until just combined.

Pour the batter into the prepared cake pan and bake for 25-30 minutes, or until the center of the cake springs back when lightly pressed. A toothpick inserted in the center should come out with a few moist crumbs.

Let the cake cool in the pan for 10 minutes before transferring it to a wire rack to cool completely. Dust with powdered sugar or top with your favorite toppings before serving.