Preheat oven to 300°F (150°C). Using a pencil, dark-colored ink pen, or permanent marker, draw an 8-inch circle on a 16 1/2 x 12-inch sheet of parchment paper. Turn parchment over, and place on a rimmed baking sheet.

In a heatproof bowl, whisk together superfine sugar, egg whites, and salt by hand. Place bowl over a saucepan of simmering water. Cook, gently whisking frequently, until sugar completely dissolves and an instant-read thermometer registers 120°F (48°C) to 130°F (54°C).

Carefully transfer sugar mixture to the bowl of a stand mixer. Using the whisk attachment, beat at medium- high speed until stiff peaks form, 4 to 5 minutes.

In a small bowl, whisk together cornstarch, vanilla bean paste, and vinegar. Add to meringue, and beat at medium-high speed until incorporated, about 30 seconds. Spoon meringue inside drawn circle on prepared pan. Using a spatula, spread meringue until it is the width of drawn template, with swoopy sides. Using spatula, make an indentation in center (about 1 inch deep). (Do not spoon out; push meringue in center toward outer edge. Final dimensions of the meringue should be 8 inches wide, 2 1/2 inches tall, and 1 inch deep in the center.) Place in oven, and immediately reduce oven temperature to 225°F (107°C).

Bake until dry to the touch, 1 1/2 hours to 1 hour and 40 minutes. Turn oven off, and let meringue stand in oven with door closed for 8 hours or overnight.

Just before serving, spoon Vanilla Mascarpone Cream in center of meringue, and top with berries. Garnish with confectioners’ sugar, if desired. Serve immediately.