Start by chopping the dark chocolate into small pieces. Place the chocolate and butter in a heatproof bowl. You can melt them using a bain-marie (a bowl set over a pot of simmering water) or in the microwave. If using the microwave, heat in short bursts, stirring in between to avoid burning.

In a separate bowl, whisk the eggs and sugar together. Whisk for a few minutes until the mixture becomes pale and slightly thickened. This step helps create a light and fluffy texture for your cake.

Sift the flour into the egg-sugar mixture to avoid any lumps. Add a pinch of salt to balance the sweetness. Whisk gently to combine everything, making sure the flour is well incorporated.

Pour the melted chocolate and butter into the egg-flour mixture. Whisk until everything is smooth and well combined. The batter should be thick, glossy, and rich.

Line a 20 cm square baking pan with parchment paper, ensuring the paper comes up the sides to make it easier to remove the cake later.

Pour the batter into the prepared pan and spread it out evenly. Place it in a preheated oven at 150°C (300°F) and bake for 20 to 25 minutes. The exact time depends on how you like the texture of your cake: 20 minutes for a runnier center, 25 minutes for a more set, fudgy texture.

Once baked, take the cake out of the oven and let it cool in the pan for a few minutes. After it has cooled slightly, carefully remove it from the pan by lifting the baking paper. Let it cool completely before slicing and serving.