Preheat your oven to 180°C and grease and line a cake tin.
Melt the dark chocolate and butter together over a bain marie, stirring until smooth.
Whisk the egg whites with the sugar until you have a glossy, stiff meringue.
Stir the egg yolks and vanilla bean into the warm chocolate mixture until fully combined.
Gently fold the meringue into the chocolate mixture in three additions, being careful not to knock out too much air.
Pour into the prepared tin and bake for 40 minutes, or until the centre has only just stopped wobbling.
Allow to cool completely before removing from the tin.
