Preheat the Oven:

• Preheat your oven to 350°F (175°C).
• Line a 38 x 26 cm (15 x 10 in) baking pan with parchment paper or grease it thoroughly.

Melt the Chocolate and Butter:

• In a heatproof bowl, combine the dark chocolate and unsalted butter.
• Place the bowl over a pot of simmering water (double boiler method) and stir until melted and smooth.
• Remove from heat, stir in the milk, and set aside to cool slightly.

Prepare the Egg Whites:

• In a large mixing bowl, add the egg whites and a pinch of salt.
• Use an electric mixer to whisk the egg whites until soft peaks form.
• Gradually add the sugar one tablespoon at a time, whisking until the egg whites are stiff and glossy.

Combine the Egg Yolks and Chocolate Mixture:

• In a separate bowl, whisk the egg yolks lightly.
• Add the vanilla extract and mix.
• Gradually pour the slightly cooled chocolate mixture into the egg yolks, stirring constantly to combine.

Fold in the Egg Whites:

• Gently fold one-third of the egg whites into the chocolate mixture to lighten it.
• Carefully fold in the remaining egg whites in batches, ensuring no streaks remain, but do not overmix to keep the batter airy.

Bake the Cake:

• Pour the batter into the prepared pan, spreading it evenly.
• Bake for 25-30 minutes, or until the center is set and a toothpick inserted comes out clean.
• Let the cake cool in the pan for 10 minutes before lifting it out.

Serve:

• Once cooled, slice and serve. Add a dusting of powdered sugar, a dollop of whipped cream, or fresh berries for garnish.