Prepare the molds: Generously grease heart-shaped molds with butter and lightly dust with cocoa powder to prevent sticking.
Melt chocolate and butter: Melt the dark chocolate and butter together using a double boiler or microwave in short intervals, stirring until smooth.
Whisk eggs and sugar: In a bowl, whisk the eggs, egg yolk, and sugar until pale and slightly thickened.
Combine wet ingredients: Slowly pour the melted chocolate mixture into the egg mixture while stirring. Add vanilla extract.
Add dry ingredients: Sift in the flour and salt, mixing gently just until combined. Do not overmix.
Fill the molds: Divide the batter evenly between molds, leaving a little space at the top.
Bake: Bake at 210°C (410°F) for 9–11 minutes. The edges should be set while the center remains soft.
Serve immediately: Let the cakes rest for 1 minute, gently unmold, dust with powdered sugar, and serve warm with desired toppings.