Combine diced strawberries, sugar, cornstarch, and lemon juice in a saucepan. Cook over medium heat 8–10 minutes until jammy. Cool completely.
In a bowl, mix flour, sugar, and salt. Cut in cold butter until coarse crumbs form.
Whisk egg, water, and vanilla; add to flour mix. Stir and knead 2–3 minutes. Chill 30 minutes.
Roll dough ⅛-inch thick. Cut 4-inch circles using cutter or bowl.
Add 1–2 tablespoon filling to each circle. Fold, seal, and crimp with a fork.
Brush with egg wash, sprinkle with coarse sugar, and cut small vents.
Bake at 375°F for 20–25 minutes until golden brown. Cool before serving.