Preheat oven to 200°F (93°C). Using a permanent marker, draw 5 (4-inch) circles 2 inches apart onto 2 sheets of parchment paper. Place parchment, marker side down, on 2 rimmed baking sheets.

For meringue: In the bowl of a stand mixer fitted with the whisk attachment, beat egg whites and cream of tartar at medium speed until foamy. Increase mixer speed to high, and add sugar, 1 tablespoon (12 grams) at a time, beating until stiff, glossy peaks form, 5 to 7 minutes. Beat in vanilla.

Spoon meringue into a pastry bag fitted with a medium open star piping tip. Following template on parchment, pipe meringue into spirals to form solid circles. Pipe 1 or 2 rings of meringue around edges of circles to create nests.

Bake until set and dry, about 2 hours. Turn oven off, and let meringues stand in oven with door closed overnight.

For filling: In a medium saucepan, heat milk, ¼ cup (50 grams) sugar, and salt over medium heat, stirring occasionally, until sugar and salt dissolve and mixture is steaming. (Do not boil.)

In a medium bowl, whisk together egg yolks, cornstarch, and remaining 2 tablespoons (24 grams) sugar. Gradually add hot milk mixture to egg yolk mixture, whisking constantly. Return mixture to saucepan, and bring to a boil over medium-low heat, whisking constantly. Cook, whisking constantly, until thickened and bubbly, 2 to 3 minutes.

Remove from heat, and strain through a fine-mesh sieve into a medium heatproof bowl. Stir in butter and vanilla until butter is melted and mixture is smooth. Cover with plastic wrap, pressing wrap directly onto surface of pastry cream to prevent a skin from forming. Refrigerate until thick and cold, at least 4 hours, or up to overnight.

In a large bowl, whisk cold cream until stiff peaks form.

Whisk cold pastry cream until smooth. Fold in whipped cream in three additions. Spoon filling into meringue nests. Top each with fruit as desired. Serve immediately.