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1
Preheat oven to 325F.
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2
With an electric mixer on medium-high speed, beat butter and confectioners sugar until pale and fluffy.
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3
Mix in 2 egg whites, 1 at a time.
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4
Mix in flour and vanilla.
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5
Spoon 1 scant teaspoon batter onto a rimmed baking sheet lined with a nonstick baking mat.
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6
Using the back of a spoon, spread into a 3-inch circle.
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7
Repeat 3 times.
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8
Bake until golden, 10 to 12 minutes.
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9
Immediately transfer cookies, 1 at a time, to inverted small brioche molds; gently press to shape.
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10
Let set, about 30 seconds.
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11
Repeat with remaining batter, baking 4 at a time.
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12
If cookies become too cool to shape, return them to oven for 20 seconds.
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13
Shells can be stored in an airtight container at room temperature up to 3 days.
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14
Reduce heat to 200F.
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15
In the bowl of an electric mixer set over (not in) a pan of simmering water, whisk remaining 3 egg whites and the granulated sugar until whites are warm to the touch and sugar has dissolved.
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16
Attach bowl to mixer; beat on medium until foamy, then raise speed to high and beat until stiff peaks form and meringue is cool, about 10 minutes.
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17
Transfer to a pastry bag fitted with a 7/16-inch star tip (such as Ateco #825).
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18
Pipe twenty 1 1/4-inch diameter, 2-inch high spirals onto parchment-lined baking sheets, spacing about 1 inch apart.
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19
Bake 20 minutes.
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20
Reduce heat to 150F.
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21
Bake until dry but not brown, about 2 hours more.
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22
Transfer to a wire rack; let cool completely.
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23
Meringue caps can be stored in an airtight container at room temperature up to 3 days.
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24
To serve, spoon 2 teaspoons lemon curd into each shell.
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25
Top each with a meringue cap.
