Preheat oven to 325 degrees. Make the shells: Put butter and sugar into the bowl of an electric mixer fitted with the paddle attachment; mix on medium-high speed until pale and fluffy. Mix in egg whites, one at a time. Mix in flour and vanilla.
Spoon 1 scant teaspoon batter onto a rimmed baking sheet lined with a nonstick baking mat. Using the back of a teaspoon, spread into a 3-inch circle. Repeat 4 times. Bake until golden, 10 to 12 minutes. Immediately transfer cookies, one at a time, to inverted small tartlet shells or brioche molds; gently press to shape. Let set, about 30 seconds. Repeat with remaining batter. If cookies become too cool to shape, return them to oven for 20 seconds. Shells can be stored in an airtight container up to 3 days.
Reduce oven temperature to 200 degrees. Make the meringue caps: Put egg whites and sugar into the bowl of an electric mixer set over a pan of simmering water; whisk until sugar has dissolved. Attach bowl to mixer fitted with the whisk attachment; beat on medium-high speed until stiff peaks form and meringue is cool, about 10 minutes. Transfer to a pastry bag fitted with a 7/16-inch star tip (such as Ateco No. 825). Pipe 20 spirals,Â 1 1/4 inches round andÂ 2 inches high, onto baking sheets lined with parchment, spacing about 1 inch apart. Bake 20 minutes. Reduce oven temperature to 150 degrees. Bake until dry but not brown, about 2 hours more. Transfer meringues to a wire rack; let cool completely. Meringues can be stored in an airtight container up to 3 days.
Make the curd: Whisk egg yolks, lemon zest and juice, and sugar in a heavy, medium saucepan. Cook, stirring constantly, over medium-low heat until mixture registers 160 degrees on a candy thermometer, 8 to 10 minutes. Remove from heat. Stir in salt. Add butter, 1 piece at a time, stirring after each addition until smooth. Pour curd through a fine sieve into a medium bowl. Cover with plastic wrap, pressing it directly onto surface. Refrigerate until cold and set, at least 1 hour (up to 2 days).
To serve, spoon 2 teaspoons curd into each shell. Top each with a meringue cap.