For crust: Within the bowl of a stand mixer fitted with the paddle attachment, beat butter, confectioners’ sugar, and salt at medium velocity till creamy, 1 to 2 minutes, stopping to scrape paddle and backside and sides of bowl. Add egg yolk; beat till nicely mixed. Beat in vanilla.
In a medium bowl, whisk collectively flours. With mixer on low velocity, step by step add flour combination to butter combination, beating simply till a dough kinds and stopping to scrape sides of bowl.
Spray 9 (2¾x1-inch) spherical tart pans with baking spray with flour; place on a rimmed baking sheet.
Divide dough into 9 parts (about 31 grams every); roll every portion right into a ball. Press a dough ball into backside and up sides of every ready tart pan; trim any extra dough. Freeze for quarter-hour.
Place oven rack in backside third of oven. Preheat oven to 400°F (200°C).
For filling: In a small saucepan, whisk collectively cheeses, granulated sugar, milk, egg, cornstarch, and salt till clean and nicely mixed. Cook dinner over medium warmth, whisking continuously, till sugar dissolves and combination is thickened and steaming. (Don’t boil.) Take away from warmth; whisk in butter and vanilla till butter is melted and combination is clean.
Pressure combination by means of a fine-mesh sieve right into a medium liquid-measuring cup with a spout, discarding solids. Divide strained combination amongst ready crusts (about 2 tablespoons or 30 grams every).
In a small bowl, whisk collectively egg yolk and 1 tablespoon (15 grams) water; gently brush on prime of filling.
Bake till crust and prime of filling are calmly browned and an instant-read thermometer inserted in middle registers 165°F (74°C) to 170°F (77°C), 10 to 13 minutes, rotating baking sheet midway by means of baking. Let cool in pans for quarter-hour. Serve heat, or let cool fully on a wire rack. Greatest served identical day as made. Refrigerate in an hermetic container for as much as 2 days.
