Preheat oven to 325°F.

Whisk egg yolks, 1 tbsp sugar and salt in a large bowl until smooth. Reserve.

Combine whipping cream and milk with remaining 3 tbsp sugar in a medium pot and bring just to a simmer. Remove from heat, add chocolate, let stand 2 minutes to melt, then stir until smooth. Stir in balsamic vinegar.

Whisking constantly, slowly add chocolate mixture to egg mixture until fully combined. Divide between 6 small ovenproof baking dishes. Cover each dish tightly with foil and place in a baking pan. Pour enough boiling water in the baking pan to come about halfway up the sides of the baking dishes.

Bake for 30 minutes or until custard is set but wiggles evenly when bumped. Serve warm, or chill until serving and serve cold.