Preheat the oven: Preheat your oven to 350°F (175°C). Grease a 9-inch round cake pan and line it with parchment paper.

1. Melt chocolate and butter: In a heatproof bowl, melt the dark chocolate and butter together over a double boiler or in the microwave in 20-30 second intervals, stirring until smooth and fully melted.
2. Beat eggs and sugar: In a separate bowl, beat the eggs and sugar together with a hand mixer or whisk until pale and frothy, about 5-7 minutes.
3. Combine wet ingredients: Add the melted chocolate mixture to the beaten eggs and sugar. Stir gently to combine, being careful not to deflate the mixture.
4. Add flavorings: Add the vanilla extract, salt, and optional baking powder, mixing until just combined.
5. Pour batter into pan: Pour the batter into the prepared cake pan and spread it evenly.
6. Bake: Bake for 20-25 minutes, or until the edges of the cake are set but the center is still slightly soft and fudgy. A toothpick inserted should come out with moist crumbs, not fully wet.
7. Cool: Allow the cake to cool completely in the pan before transferring it to a serving plate.
8. Finish: Dust with powdered sugar for an elegant finish, if desired. Slice and serve!