To Make the Crust:

1. Whisk together the flour, sugar, and salt. Carefully (watch those fingers) and quickly use a cheese grater to grate the cold butter into the flour mixture — as anyone who's watched The Great British Bake Off knows, you gotta keep that butter cold. Cut the butter into the flour using a pastry cutter (or bench scraper) until you have large crumbles.
2. Add the sourdough starter discard and vinegar, using a fork to combine the dough (remember, you're trying to avoid melting the butter). If it's too dry and crumbly, add a teaspoon of ice water at a time until it's cohesive, but be careful not to overdo it. Shape the dough into a rectangular shape.
3. Wrap the dough in plastic wrap, then use a rolling pin to flatten it a little and give it a nice smooth shape. Chill it in the fridge for at least one hour but up to 24 hours if you want to prep it early!

To Assemble:

1. Preheat your oven to 350°F, and line a baking sheet with parchment paper.
2. Lightly flour your surface, then roll the dough into a rectangle, with around a 1/8-inch thickness. Cut the dough into 4 strips, and brush all around the edges with the egg wash.
3. Mix together the brown sugar, ground cinnamon, and flour for the filling.
4. Add a heaping spoonful 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 on top of each pop tart.
5. Place the pop tarts onto the baking sheet, and bake for 20-25 minutes (watch for a golden brown crust). Transfer to a cooling rack to glaze.

To Glaze:

1. Whisk the powdered sugar and water together until it's smooth and spreadable (you may need to add a little extra water). Spoon onto each pop tart while it's still warm, and sprinkle with coarse sugar or sprinkles.