Preheat the oven to 275°F or 250°F if your oven runs hot. Do not use convection. Line a baking sheet with unbleached parchment paper (it’s best to use parchment paper for this recipe as opposed to a silicone mat to remove any trace of residual oil that can affect the pavlova). Using a pencil or dark marker, draw an 8-inch diameter circle on the parchment paper by tracing around an 8-inch cake pan or plate. Flip the paper over so your meringue won't touch the marker.
Dampen a paper towel with a little vinegar and wipe the inside of a mixing bowl (metal, ceramic or glass - not plastic) and your beaters or whisk attachment to remove any trace of residual oil. Then wipe with a clean paper towel. Don’t worry about the vinegar smell, it won’t affect the final result.
In the prepared bowl of your electric mixer fitted with the whisk attachment, beat the egg whites on medium speed until foamy, about 2 minutes. Raise the speed to medium-high and gradually add the sugar, 1 Tablespoon at a time. I like to count to 10 in between each addition. Continue beating until the meringue is glossy and stiff peaks form, 8-10 minutes.
Sift in the arrowroot and cream of tartar (if you are using vinegar and/or vanilla, they go in now) and beat another minute until combined.
Mound the meringue onto the parchment inside the circle. Using the spatula or a spoon, spread the meringue to fill the circle. Create a bit of a well in the center. That’s where the cream will go for serving. Place in the oven right away (no sitting out and waiting) and bake for 75 minutes at 275°F or 90 minutes at 250°F, or until the meringue is puffed and crisp all over, yet still not totally firm underneath if you touch the center. Do not open the oven door. Changes in temperature are bad for meringue. Cracks are totally normal and part of the charm. Turn off the oven, open the oven door a crack, like with a wooden spoon, and leave the meringue in the oven to cool to room temperature, at least 1 hour. I usually leave it overnight.
Before serving, place a large bowl and the beaters of an electric mixer in the freezer. Carefully peel the meringue off of the parchment paper and place it on a serving platter.
Prepare the whipped Greek yogurt and cream: Remove the mixing bowl and beaters from the freezer. Place the cream and yogurt in the bowl and beat on high speed. While the mixer is going, add the sugar and vanilla and beat until soft peaks form. Check the cream after 3 or 4 minutes and be sure not to overbeat.
When you are ready to serve: Mound the whipped cream onto the meringue and gently spread it out about 1 inch from the edge (don't worry if the meringue cracks in the process). Top the pavlova with the berry sauce and/or berries. Cut the pavlova into wedges, wiping the knife in between slices if necessary, and serve.