Begin by preheating your oven to 350°F and greasing a large baking mold to prevent sticking. In a double boiler, melt 225 g of dark chocolate with 100 g of unsalted butter, stirring occasionally until the mixture is smooth and fully combined. While the chocolate melts, separate your 4 large eggs and place the yolks in a separate bowl.
Whisk the egg yolks together with 175 g of sifted powdered sugar, beating until the mixture becomes pale and creamy—this step is essential for incorporating air into the batter. Once the chocolate and butter have melted, carefully fold this mixture into the egg yolk and sugar combination, using a spatula to gently combine without deflating the mixture.
In a clean, dry bowl, whisk the 4 egg whites until soft peaks form—peaks that gently curl over when the whisk is lifted. This step is pivotal for creating the cake's characteristic light, airy texture. Carefully fold the whipped egg whites into the chocolate mixture in two additions, using gentle folding motions to maintain the airiness of the whites while ensuring even distribution throughout the batter.
Pour the batter into the prepared baking mold and place it in the preheated oven. Bake for approximately 30 minutes, watching for a slightly cracked surface that indicates the cake is set on the edges. The center should still appear gooey and underbaked—this is intentional and creates the signature fudgy texture that makes this cake so indulgent. Allow the cake to cool slightly before serving, as the residual heat will continue to set the interior slightly.
