Separate egg yolks and whites.
Place egg whites in a mixing bowl and allow them to come to about room temperature. (You can put them in a small bowl and put that bowl in the bottom of your sink filled with a bit of warm water to speed it up)
Preheat oven to 350 degrees F (130 degrees C) and place rack in center of oven. Line a baking sheet with parchment paper and draw an 8 or 9 inch (20 cm) circle on the parchment paper using a pan as your guide. You do not need to cut it out, just turn it over so the pencil doesn't get onto the food.
In the bowl of your electric mixer with the whisk attachment, beat the egg whites until foamy. Add the cream of tartar and continue to beat on medium-high speed until they hold soft peaks. Start adding the sugar, a little at a time, and continue to beat until the meringue holds stiff peaks. (Test to see if the sugar is fully dissolved by rubbing a little of the meringue between your thumb and index finger. The meringue should feel mostly smooth, not too gritty. If it feels super gritty the sugar has not fully dissolved so keep beating until it feels smooth between your fingers).
Using a large rubber spatula, gently fold in the cornstarch, vinegar, vanilla extract, cocoa powder, and chocolate pieces. It's important to fold and not stir to preserve the air in the egg whites.
Gently spread the pavlova batter inside the circle drawn on the parchment paper, smoothing the edges, and any small peaks. The pavlova will keep almost the exact shape after being baked.
Place Pavlova in the oven and IMMEDIATELY turn oven temp down to 250 deg F.
Bake for about 1 hour and 5 minutes or until the outside of the pavlova feels firm to the touch if gently pressed. There will be cracks and you will see that the inside is soft and moist. Turn the oven off, leave the door ajar, and let the meringue cool completely to room temperature in the oven.
The cooled meringue can be made and stored in a cool, dry place, or in the refrigerator in an airtight container, for a few days before it is topped. I personally prefer to eat it slightly chilled when the chocolate pieces have become firm again.