Preheat the oven to 200°F (93°C). In a stand mixer bowl, combine egg whites, kosher salt, and cornstarch. Whip on medium speed until foamy, about 2–3 minutes.

Gradually add sugar, one tablespoon at a time, while whipping on high speed until stiff peaks form, around 5–7 minutes.

Gently fold in vinegar and vanilla extract. Mix until smooth.

Pipe meringue into small circles on a lined baking sheet, creating a cavity in the center of each.

Bake for 2 hours until dry and crisp. Cool in the oven with the door ajar.

In a saucepan, bring lemon juice to a simmer, reducing by half. In a mixing bowl, beat butter and powdered sugar until fluffy.

Add eggs and yolks to the butter mixture, followed by a pinch of salt.

Slowly whisk in reduced lemon juice. Cook over a double boiler for 10-15 minutes until thickened.

Strain the curd into a bowl, let cool, then refrigerate for at least 1 hour.

Fill each meringue with chilled lemon curd, garnish with berries and mint. Dust with powdered sugar.