In the bowl of a stand mixer (a hand mixer will work too, it will just take ages), add in the egg whites, sugar, and salt. Whisk to combine.
Place a medium sauce pot with 2" of water onto the stove. Bring the water to a simmer.
Set the mixing bowl with the egg whites and sugar over the bowl of simmering water and turn the heat to low. The water shouldn’t touch the bottom of the mixing bowl.
Whisk the egg whites and sugar constantly until the sugar has fully dissolved into the egg whites. To check and see if the sugar has dissolved, pinch and rub some of the mixture in between your fingers and feel if you still feel any of the sugar granules, if not you’re good to go. If you do just keep cooking a bit longer.
Once the sugar has dissolved, turn off the heat and remove the bowl from the water. Leave it to cool off for about 10 minutes.
Using your mixer, whip the egg white mixture first on medium and then on high speed until fluffy, shiny and stiff.
Add in the vanilla bean paste (vanilla extract will work too!) and whip again until combined.
Once set, remove the plastic wrap from the surface of the tart and carefully press the tart base up and out of the shell.
Loosen along the edges where the base and edges of the tart shell connect, if you press just from the center you have a higher chance of the crust breaking.
If you're nervous about removing it from the base you can always leave the tart on the pan base.
Either with a piping bag and tip or a big spoon, top the tart with the meringue.
If you have a kitchen torch you can lightly toast the surface of the meringue.
Slice with a big sharp knife straight down through the tart. Clean the knife between each cut to get beautiful clean slices.