Preheat oven to 350°. Line a baking sheet with parchment paper. In stand mixer with whisk, beat egg whites on medium-high speed to form soft peaks, approx. 5 min. Add sugar slowly over the course of 1 min, then turn to high speed for 2 min more. Add vanilla, mixing on high speed 3-5 min until sugar is dissolved and meringue is very stiff and glossy. Add corn starch and cream of tartar, blending very briefly. Pour out onto parchment-lined sheet pan, shaping gently into an approx. 8-inch circle. Pull edges up slightly with back of a spoon or spatula, and add a few peaks around top, leaving a slight dip in center for topping later. Place meringue in oven and reduce heat to 200°. Bake until pavlova is firmly set, 1½ hours. Turn oven off and let meringue cool in oven at least one hour more, until dry to the touch. Remove from oven and cool to room temp. Store in airtight container if serving later. To serve, top with Berry Topping (see below), fresh whipped cream and fresh berries.
For berry topping
Combine all ingredients in saucepan on stovetop over medium-high heat until bubbling. Remove from heat and let cool to room temp before topping pavlova.