You need a heatproof bowl and a pot filled ⅓ full with water; the bowl should be able to sit on top of the pan without touching the water. Do not put the bowl on top of the pot yet.
Bring the water to a simmer. Set the egg whites in the bowl then grab a whisk and pour the sugar in, whisking as you do. Whisk until it’s fully mixed then place over the simmering water.
Whisk constantly, alternating between slow and quick, as the bowl warms then the egg mix warms. Switch to a rubber spatula to scrape the sides of the bowl so all the mix is at the bottom (otherwise those sugar granules sticking to the sides will not dissolve and will go into your meringue).
Lift the spatula, dragging some of the egg up so it pours down and, between two fingers, pinch some that’s falling back into the pot. This is to check on the sugar and see if you can detect any sugar granules.
After about 4-5 minutes, you should be able to do this and not detect any sugar granules, remove it from the heat and pour it into the bowl of a stand mixer. Affix the whisk attachment and begin whipping on low, gradually increasing speed to medium high (about a 6 on a kitchenaid). Whip for about 10 minutes, checking once the meringue looks thick and glossy. Add the vanilla and salt after about 7 minutes, while the mixer is on.
To check it, remove the whisk and turn it upside down: does the meringue droop or hold its shape? Once it holds stiff it is done. If you are adding cocoa or berry powder, stop before it’s completely stiff and sift the powder in then use a rubber spatula to gently fold the powder in until it’s mostly incorporated.