In a medium-sized mixing bowl, whisk together the flour, sugar, and salt.
Grate the cold butter into the flour mixture and use a bench scraper or pastry cutter to incorporate it until the mixture forms large crumbles.
Add the sourdough discard and vinegar, mixing with a fork until the dough starts to come together. If necessary, add a teaspoon of cold water to adjust consistency.
Shape the dough into a rectangle, wrap it in plastic wrap, and flatten it with a rolling pin.
Chill in the fridge for at least 1 hour (up to 24 hours).
Preheat your oven to 350°F (177°C)
Line a baking sheet with parchment paper.
Roll out the chilled dough on a lightly floured surface to about ⅛" thick, cut into 4 long strips, and brush edges with an egg wash.
Place 2 tablespoons of strawberry jam on one side of each strip, fold the dough over and use a fork to press and seal the edges.
Brush the top of each pop tart with the egg wash and bake for 25-30 minutes until golden brown. Transfer to a cooling rack.
Mix the powdered sugar and water until smooth.
Drizzle over the cooled pop tarts and add sprinkles or coarse sugar as desired.
