In a medium-sized mixing bowl, whisk the flour, sugar and salt.
Grate the cold butter into the flour mixture.
Use a bench scraper or a pastry cutter to cut the butter into the flour until it forms large crumbles.
Add the sourdough starter discard and vinegar to the mixture and use a fork to bring the dough together.
Use your hands to bring the mixture into a cohesive ball and press into a rectangular shape.
Wrap the dough in plastic wrap and use a rolling pin to flatten the dough and smooth the edges.
Let the dough chill in the fridge for at least one hour.
Preheat your oven to 350°F (177°C) and line a baking sheet with parchment paper.
Roll the dough into a rectangle, with a ⅛" thickness, on a lightly floured surface.
Cut the dough into 4 long strips.
Brush the edges with the egg wash.
Add 2 tablespoons of filling to one side of each rectangle and fold the dough over itself.
Use a fork to press and seal the edges.
Brush the egg wash over the top of each pop tart.
Place the pop tarts onto the baking sheet and bake for 25-30 minutes or until the crust is golden brown.
Remove from the oven and transfer the baked pop tarts to a cooling rack.
In a small bowl, whisk the powdered sugar and water together until thick and smooth.
Pour onto each pop tart and sprinkle with coarse sugar or sprinkles.