In a medium saucepan, bring plums, sugar, lemon juice, and salt to a boil over medium heat, stirring frequently; cook, stirring occasionally, until plums begin to soften and release juice, about 5 minutes. Stir in strawberries.

Carefully transfer about ¼ cup (60 grams) juice from pan to a small bowl; whisk in cornstarch until smooth. Stir cornstarch mixture into fruit mixture; cook, stirring frequently, until mixture is thick and translucent, 2 to 3 minutes. Remove from heat, and stir in vanilla. Let cool completely.

Line a baking sheet with parchment paper.

On a lightly floured surface, roll All-Butter Piecrust to ⅛-inch thickness. Using a 4-inch round cutter, cut dough, rerolling scraps once. Spoon 1 tablespoon (20 grams) cooled fruit mixture into center of each round.

In another small bowl, whisk together egg and 1 tablespoon (15 grams) water; brush onto edges of dough. Fold dough in half over filling, pressing edges together to seal. Using a fork, crimp as desired. Place pies on prepared pan. Refrigerate until firm, at least 20 minutes.

Preheat oven to 375°F (190°C).

Brush egg wash onto dough. Cut a few small slits in dough to let steam escape.

Bake until golden brown, about 15 minutes. Let cool on pans for 10 minutes. Serve warm, or let cool completely on wire racks. Store in an airtight container for up to 3 days.