Preheat oven to 300°F. Line a baking sheet with parchment and trace an 8-inch circle; flip the paper over.
Beat egg whites in a clean, dry bowl on medium speed until soft peaks form.
With the mixer running, add sugar a spoonful at a time. Beat until thick, glossy, and at stiff peaks.
Sift in cornstarch and salt. Add vinegar and 1 tsp vanilla paste. Fold gently until just combined.
Spoon meringue inside the circle. Spread into a disk with a shallow center well and slightly higher edges.
Place the tray in the oven and immediately reduce heat to 225°F. Bake 75 minutes until dry to the touch.
Turn the oven off, crack the door, and cool the pavlova inside for 20–25 minutes. Transfer to a rack to finish cooling.
Toss berries with the optional tablespoon of sugar and let stand 10 minutes.
Whip cream with powdered sugar and 1/2 tsp vanilla paste to soft peaks.
Just before serving, fill the pavlova with whipped cream and top generously with berries. Slice and serve.