In a heatproof bowl, combine egg whites, sugar, and salt, and set mixture over a saucepan of simmering water, keeping heat on low. Constantly yet gently whisk mixture by hand until warm to the touch and smooth, and sugar has dissolved, about 5 minutes. If you have a thermometer, the temperature should reach 160°F/71°C. Remove from heat.
Using an electric mixer fitted with the whisk attachment, whisk egg white mixture until stiff peaks form and meringue is thick and glossy, about 6-8 minutes (start with low speed and gradually increase to medium-high), adding in vanilla extract near the end.
Spread meringue gently over chocolate filling and swirl it decoratively using the back of a spoon. Toast topping using a kitchen torch. Place in the fridge until serving.
Tart will keep in an airtight container for up to 5 days in the fridge.