In a large bowl combine dry ingredients (flour, sugar, and salt.) Cut in the cold butter with a pastry cutter into the flour mixture until the butter is in small pieces, about the size of peas.
Combine sourdough discard, egg, and cold water. Then add to flour mixture. Stir until loose pastry dough. Turn out onto a lightly floured surface and work it into a ball. Depending on how dry your dough is, you may need to add another tablespoon of water of two. Temperature and how you measure out your flour can affect how dry your dough is. I scoop and level my flour measurements.
Wrap in piece of plastic wrap and chill for about 30 minutes.
After dough has chilled, roll out your sourdough pie dough into a large rectangle (about 16x20 inches) on a floured work surface with a floured rolling pin. I cut all round the edges to made it straight. You can either take all the scraps and roll them out and make more pop tarts or you can just sprinkle them with cinnamon and sugar and make cinnamon sticks!
Cut pie dough in half with a bench scraper or sharp knife. Next cut strips about 3 inches wide for your pop tarts.
Put a rounded Tablespoon of the filling on each bottom half of the pop tart and spread out on about half of the pre-cut pop tart, leaving room to seal all the edges.
Brush all edges with egg wash to seal the edges. Fold over the top of pop tart onto the filling. Press the edges down and then use the tines of a fork to crimp the edges. Place on a baking sheet lined with parchment paper or a silicone mat. Bake in 400 degree preheated oven for 15 minutes, until they start turning golden.
Place on a cooling rack to cool. Enjoy