Make the Pavlova: Preheat the oven to 250 degrees F. Line a baking sheet with parchment paper. Beat the egg whites, cream of tartar and salt in a large bowl with a mixer on medium-high speed until very foamy, about 2 minutes. Gradually beat in the sugar, about 2 tablespoons at a time (this should take about 3 minutes). Continue beating until stiff and shiny, about 3 more minutes.
Sift the cocoa powder over the egg white mixture and sprinkle with the chopped chocolate; fold to combine. Spread the meringue into an 8-inch circle on the prepared baking sheet, making a shallow well in the center. Reduce the oven temperature to 225 degrees F; bake until firm and dry, about 2 hours. Turn off the oven and open the door slightly. Let the meringue cool in the oven, about 3 hours. Carefully remove from the parchment and transfer to a platter.
Make the toppings: Beat the heavy cream, sour cream, vanilla and 2 tablespoons sugar in a large bowl with a mixer on medium-high speed until soft peaks form, about 2 minutes. Toss the strawberries with the remaining 1 teaspoon sugar. Top the Pavlova with the whipped cream and strawberries.