Pavlova

• 1/2 lemon (optional)
• 4 large egg whites, at room temperature
• Pinch of fine sea salt
• 1 cup superfine sugar
• 2 teaspoons cornstarch
• 1 teaspoon white vinegar
• 1/2 teaspoon vanilla extract

Topping

• 1 (8 3/4-ounce) can sweetened chestnut spread (such as Clement Faugier)
• 1/8 teaspoon fine sea salt
• 1 1/2 cups heavy cream
• 1 small bar bittersweet (70% cacao) chocolate, for shaving