Preheat the oven to 350°F (177°C). Line a baking sheet with parchment paper. Draw a 9-inch (23-cm) circle on the parchment paper and then flip the parchment paper over. *Take note that you'll reduce the oven temperature to 200°F (93°C) as soon as you put the pavlova in the oven.
Add the egg whites and sugar to a large bowl (make sure it's heat-safe) and set atop a saucepan of simmering water over medium-high heat. Do not allow the bottom of the bowl to touch the water. Whisk until the mixture is thick and frothy, the sugar is melted (rub some between two fingers; if it feels gritty, keep whisking). This should take about 3 to 5 minutes.
Remove the bowl from the heat, transfer the sugar-eggs to the bowl of a stand mixer fitted with the whisk attachment and add the cream of tartar and salt. Mix on medium speed until you have stiff glossy peaks and the meringue is cool to the touch, about 8 minutes (don’t rush it). Add the confectioners’ sugar and vanilla and whisk on high for 1 minute more until stiff peaks return. When inverted, the tip of the meringue should point just off to the side (not quite straight up).
Sprinkle the cocoa powder, espresso powder and chocolate over the top of the meringue and give it 2 or 3 folds. It will not be combined. Spoon meringue onto center of the circle you drew on the parchment paper (on the prepared baking sheet). It will be a bit messy with loose chocolate and cocoa powder, but that's ok (see video). Use an offset spatula or the back of a spoon to sculpt the meringue into a round, flat disc that fits inside the 9-inch (23-cm) circle you drew. Don’t overwork or over-mess with the meringue. It may look messy but that’s it’s magic.
Place the pavlova in the center of the preheated oven and immediately reduce the oven temperature to 200°F (93°C) and bake for 2 hours. Once done, let it sit in the turned-off oven with the door closed until it has cooled to room temperature. When done, it should have a crisp outer shell and marshmallowy center. It will take several hours to cool and become the correct consistency. Leave in the turned-off oven until ready to serve.