Preheat oven to 350°F. Line a baking sheet with parchment paper; draw an 8-inch circle on center of paper. Flip paper over; set aside.
If desired, rub the inside of a large bowl or bowl of a stand mixer with cut side of lemon half to ensure that the bowl is grease-free. Add egg whites and a pinch of salt to bowl. Beat with an electric mixer or stand mixer fitted with the whisk attachment on medium-high speed until mixture forms firm but not stiff peaks, about 2 minutes. With mixer running, add sugar, 1 tablespoon at a time, beating well after each addition (about 30 seconds per addition) until mixture is stiff and glossy, about 10 minutes. Be patient: The trick to making a pavlova base is to whisk fast and add the sugar slowly.
Add cornstarch, vinegar, and vanilla to egg white mixture. Use a rubber spatula to gently fold and incorporate the ingredients.
Transfer mixture to prepared baking sheet and form a mound in center of hand-drawn circle. Using an offset spatula, shape meringue into an 8-inch round, about 1 1/2 inches tall, with a flat top and straight sides, using the circle as your guide.
Place baking sheet in preheated oven and immediately reduce oven temperature to 300°F. Bake until pavlova is puffed, beginning to crack on top, and feels crisp when gently tapped, about 1 hour. Transfer pavlova on baking sheet to a wire rack and let cool completely, at least 2 hours or up to 6 hours. (Pavlova will deflate and crack as it cools.)
Just before serving, carefully invert pavlova base onto a platter so that cracked side is now on the bottom. Peel off and discard parchment paper.
Stir together chestnut spread and salt in a medium bowl until smooth and spreadable, about 1 minute. Gently spread mixture over top of pavlova base, spreading all the way to edges, forming a thicker border that will be visible after topping with whipped cream.
Beat cream with electric mixer or stand mixer with whisk attachment on medium-high speed until medium peaks form, 1 minute and 30 seconds to 2 minutes. Gently spread whipped cream on chestnut mixture, leaving a thin border (1/4 to 1/2 inch) of chestnut mixture visible. Using a large Microplane shaver, shave some chocolate into long splinters over whipped cream. Serve immediately.
