Preheat your oven to 375 degrees F.
Add the flour and salt to the food processor and pulse to mix. Then add the grated frozen butter to the flour mixture and pulse until crumbly.
Add the sourdough discard and pulse again until the dough forms a ball. (you can use this dough right away, or you can refrigerate it for up to 48 hours, wrapped tightly in plastic wrap for a long-fermented dough.)
On a lightly floured work surface, roll out he dough with a floured rolling pin. Roll into a rectangle shape about 1/8" thick.
Try to make a perfect rectangle by trimming the pastry dough around the edges. Then using a bench scraper or a knife, cut the dough into long strips.
Scoop out 1-2 tablespoon of jam onto the end of the strip, leaving room around the edges. Wet your finger and run it along the edges of your pop tart to seal it up, then fold the dough over. Pinch closed and press around the edges with the tines of a fork.
On a baking sheet lined with parchment paper, place the unbaked pop tarts. You can add an optional egg wash by brushing on 1 egg yolk mixed with 1 tbsp of water.
Bake for 20-25 minutes, until the edges are golden brown and then transfer to a cooling rack.
Prepare the glaze while the pop tarts are cooling by adding all the ingredients into a small saucepan on medium low heat. Stir until smooth with no lumps, you may want to use a whisk.
Once the homemade pop-tarts are cooled, add the glaze over top, allow to harden up a bit, and enjoy!