Preheat oven to 225°F with racks in upper and lower third positions. Line 2 large baking sheets with parchment paper. Whisk together sugar and cornstarch in a bowl; set aside. Beat egg whites with an electric mixer fitted with a whisk attachment on medium-high speed for 1 minute. Add cream of tartar and salt; beat until combined, about 30 seconds. With mixer running on high speed, add sugar mixture 2 tablespoons at a time, beating just until glossy, stiff peaks form and sugar is almost dissolved, about 2 minutes. (Do not overbeat.) Reduce speed to low, and beat in ½ teaspoon of the vanilla.

Gently spoon about ⅓ cup egg white mixture onto prepared baking sheet in a 3 ½-inch circle. Repeat with remaining egg white mixture, creating 12 circles total on the 2 prepared baking sheets.

Bake in preheated oven until meringues have formed a crust, about 1 ½ hours, rotating baking sheets between top and bottom racks halfway through bake time. 

Turn oven off. Let meringues stand in oven with door closed until completely cool with dry and shiny tops, at least 3 hours or up to 10 hours, depending on the humidity of your kitchen. (They should have crisp exteriors and chewy interiors.)

Meanwhile, chill bowl of a stand mixer and whisk attachment for 30 minutes. Pour whipping cream into chilled bowl; beat with an electric mixer fitted with the chilled whisk attachment on medium-high speed until soft peaks form, 2 to 3 minutes. Add sour cream, and beat until stiff peaks form, about 30 seconds. Beat in remaining 1 teaspoon vanilla until just combined.

Arrange meringues on a serving platter, and top each with about ¼ cup whipped cream mixture. Garnish Pavlovas with toppings.