Buttery, flaky sourdough pastry envelopes a bright, jammy center, then gets finished with a delicate vanilla-sweet glaze for a nostalgic breakfast treat elevated by the tang and depth of sourdough discard.
In a large mixing bowl, whisk together the flour, salt, and sugar.
Grate or cut in the cold butter until pea-sized pieces remain; do not overmix.
Add the sourdough starter discard and mix with a fork just until the dough comes together.
Divide dough into two, shape into discs, wrap tightly, and chill for at least 1 hour (up to 3 days).
On a lightly floured surface, roll one disc into a large rectangle about 1/8 inch thick; trim edges and cut into 4 equal long rectangles.
Spoon jam onto one end of each rectangle, leaving a small border; fold the dough over the filling and crimp edges with a fork. Transfer to a parchment-lined baking sheet. Repeat with the second disc.
Beat the egg and brush the tops of the pop tarts with the egg wash.
Bake at 350°F (175°C) for about 30 minutes, or until golden brown.
While they bake, whisk the powdered sugar and milk into a smooth glaze.
Let pop tarts cool completely, then spoon the glaze over the tops and allow it to set before serving.