Three-Layer Berry and Brown Sugar Pavlova

INGREDIENTS

• Confectioners sugar for dusting
• 1/2 cup packed light brown sugar
• 2 teaspoon distilled white vinegar
• 3/4 cup egg whites (from 5 to 6 large eggs) at room temperature 30 minutes
• 1 1/2 pounds strawberries, trimmed and quartered
• 1 1/2 tablespoons cornstarch
• 1 1/2 teaspoons pure vanilla extract
• 1 cup chilled heavy cream
• 2 tablespoons granulated sugar
• 1/3 cup chilled sour cream
• 1 pound blackberries
• 1 cup superfine granulated sugar