01 - Preheat the oven to 225°F. Line two baking sheets with parchment paper.
02 - In a clean, dry bowl, beat the egg whites and salt with an electric mixer on medium speed until foamy. Add the cream of tartar.
03 - Gradually add the superfine sugar, one tablespoon at a time, beating on high until stiff, glossy peaks form. Beat in the vanilla.
04 - Spoon or pipe eight mounds of meringue onto the prepared baking sheets, creating a slight well in the center of each.
05 - Bake for 1 hour 15 minutes, then turn off the oven and let the meringues cool completely inside the oven for about 30 minutes.
06 - Meanwhile, in a heatproof bowl, whisk together egg yolks, sugar, lemon juice, orange juice, and both zests.
07 - Place the bowl over a saucepan of simmering water and cook, stirring constantly, until thickened for about 8-10 minutes.
08 - Remove from heat and whisk in the butter until smooth and glossy. Let the curd cool to room temperature.
09 - Once the meringues are cool, carefully remove from parchment. Spoon the citrus curd into the centers.
10 - Garnish with fresh berries, edible flowers, or mint if desired. Serve immediately.