Preheat your oven to 200°F (95°C) and line two baking sheets with parchment paper. Beat the egg whites with kosher salt and cornstarch until foamy peaks form, then gradually add granulated sugar until stiff, glossy peaks form. Fold in vinegar and vanilla extract.

Dollop or pipe the meringue onto the baking sheets, creating small nests. Bake for 2 hours until completely dry. Turn off the oven and allow meringues to cool slowly inside.

While meringues cool, prepare the lemon curd. Simmer fresh lemon juice until reduced by half, then cream butter and sugar, add egg yolks, and mix well.

Pour the lemon curd mixture into a heatproof bowl over simmering water, stirring until it thickens and reaches 166°F (75°C), about 8-10 minutes. Strain through a fine sieve and refrigerate until chilled.

Once cooled, fill each meringue nest with lemon curd, topping with fresh berries and mint.

Dust with powdered sugar for an elegant finish and serve or store in an airtight container.