In a small saucepan, whisk jam, cornstarch, and water. Heat on medium for 2 minutes until slightly thickened. Let cool.
Preheat oven to 375°F (190°C). Roll one crust on a parchment-lined baking sheet. Spread filling, leaving a 1-inch border. Top with the second crust and seal edges with a fork.
Brush the top with beaten egg, sprinkle sugar, and bake for 25–30 minutes until golden. Cool completely.
Whisk powdered sugar, milk, and vanilla into a glaze. Drizzle over the pie and add sprinkles.
Slice into squares or wedges and serve fresh.