Preheat oven to 275°.
To prepare pavlovas, cover a baking sheet with parchment paper. Draw 8 (4-inch) circles on paper. Turn paper over; secure with masking tape.
Sift together cocoa and potato starch; set aside.
Place salt and egg whites in a large bowl; beat with a mixer at high speed until foamy. Gradually add 1 cup sugar, 1 tablespoon at a time, beating until stiff peaks form. Beat in vinegar and 1 teaspoon vanilla. Sprinkle cocoa mixture over surface of egg white mixture; beat at low speed just until incorporated, scraping sides of bowl.
Spoon about 1/2 cup egg white mixture into each drawn circle on prepared baking sheet. Spread egg white mixture onto the circles using the back of a spoon; make an indentation in the center of each meringue circle to form a nest. Bake at 275° for 1 hour. Turn oven off; cool meringues in closed oven at least 12 hours or until completely dry. Carefully remove meringues from paper. Cool completely on a wire rack.
To prepare strawberries, combine berries and remaining ingredients in a bowl; toss gently. Cover and chill at least 1 hour; toss gently before serving. Place 1 meringue circle on each of 8 plates. Top each serving with 1/2 cup berry mixture. Garnish with mint sprigs, if desired. Serve immediately.