In a medium bowl, whisk together the flour, salt, and sugar.
Cut in the cold butter using a pastry cutter or your fingers, or a food processor until the mixture resembles coarse crumbs. You could also use a cheese grater to grate your butter for a flakier sourdough pie crust.
Cut in the cold butter using a pastry cutter or your fingers, or a food processor until the mixture resembles coarse crumbs. You could also use a cheese grater to grate your butter for a flakier sourdough pie crust.
Add sourdough discard and mix until a dough forms. If needed, add 1 tablespoon cold water at a time until the dough holds together. The dough should come together without being "wet".
Divide the dough in half, shape into rectangles, wrap, and refrigerate for 1 hour (or overnight for a long fermentation--see tips).
On a floured surface, use a rolling pin to roll out half of the tart dough into a large rectangle shape (about ⅛ inch thick).
Trim edges with a sharp knife and cut into even rectangles (around 3" x 4"). Use a bench scraper if needed.
Place half of the rectangles on a lined baking sheet. Add 1 tablespoon jam to the center of each, leaving space around the edges.
Top with remaining rectangles. Use a fork to crimp edges and seal. Poke the tops gently with a fork to let steam escape.
With a pastry brush, brush the egg wash over the tops of the unbaked pop tarts. Preheat oven to 350°F. Bake for 20–25 minutes or until golden brown.
Let the pop tarts cool slightly on a cooling rack. Mix glaze ingredients in a small bowl until smooth and drizzle over top of the pop tarts. Add sprinkles if you're feeling festive!