For the Pavolva
Preheat oven to 300°F and line a small sheet tray with parchment paper. Whip egg whites and cream of tartar with a whisk, beater, or whip attachment until frothy, about 6 minutes.
Turn off the mixer and sprinkle in half of the sugar, the cornstarch, and the vanilla extract, and whip for 1 minute on medium speed. Turn off mixer again, add the remaining half of the sugar, and resume whipping on high for 3 to 5 minutes or until peaks are stiff and glossy.
Using a rubber spatula or large spoon, heap six even piles of meringue onto the parchment-lined sheet tray. Artfully smooth the globs with a gentle swirling motion.
Place meringues into the oven and drop the heat to 200°F. In my oven at home, I keep it at that temperature for an hour and a half before turning the oven off and leaving them inside to cool and dry. In the commercial oven at Pomet, I turn off the oven immediately and leave them inside to cook and dry. It may take some practice to learn your oven, but if the meringues start to look toasted, or if they crack and weep sugar, those are indications that the temperature is too hot.
For the Blackberry-Oolong Whipped Cream Filling
If possible, toss the blackberry pieces in sugar the day before and leave to macerate at room temperature. This brings out the juices and helps them to jam nicely. Otherwise, do it the day of and be super vigilant while cooking them, adding water as necessary to avoid scorching.
After the berries have macerated in sugar, place them in a small pan on low heat and cook down to a jammy consistency, pressing them with the back of a fork to coax out all the juice. Cool in the fridge.
In a clean small pan, bring the heavy cream and tea leaves to a near boil, then steep 3 minutes. Strain and chill the mixture.
Once both the jam and the heavy cream are cold, whip together to medium-stiff peaks. Reserve in refrigerator.
To assemble the pavlovas: Once you’re ready to serve these, place one pavlova on each plate. This is the fun part: Use the back of a spoon to pop the top of the meringue, breaking it up and revealing the soft marshmallow-y interior. Spoon a generous amount of cream into the hole you just made. Garnish with blackberry halves, pieces of fresh lemon thyme, and a dusting of ground oolong.
These aren’t meant to sit around, so once they’re assembled, enjoy immediately while all the distinct textures are still … well … distinct.