Preheat oven to 350°F. Grease and line a round cake pan. Melt chocolate and butter together until smooth. Whisk in sugar and vanilla. Add egg yolks one at a time. Beat egg whites with salt until soft peaks form. Gently fold egg whites into chocolate mixture. Pour batter into pan. Bake 25–30 minutes until edges set and center stays soft. Cool completely before removing from pan.