Prepare the Dough:

In a large mixing bowl, combine the flour and cold butter using a pastry cutter or food processor until the mixture forms large crumbles.
Sprinkle in the salt and sugar, and mix again.
Gradually add the sourdough discard in three phases, mixing after each addition until the dough is slightly shaggy.
Drizzle in the apple cider vinegar, then add the teaspoon of ice water and vanilla. Mix until the dough starts to come together, though it will still be chunky at this point.
Knead the Dough:

Turn the dough out onto a lightly floured surface and knead gently until it forms a cohesive dough. Be sure to press any large chunks of butter into the dough.
Divide the dough into two squares, wrap in plastic wrap, and refrigerate for at least 1 hour, or up to 24 hours for a longer ferment.
Roll Out and Shape:

Preheat your oven to 350°F (175°C).
Roll out the dough on a lightly floured work surface until it's about 1/4 inch thick. Use a pizza cutter to cut the dough into small rectangles or long strips for your pop tarts.
Brush the edges of each rectangle with egg wash.
Fill and Seal:

Place about 2 tablespoons of your chosen filling in the center of each rectangle.
Fold the dough over and gently press out any air from the creased side.
Crimp the edges with a fork to seal, ensuring none of that sweet filling escapes.
Bake:

Place the unbaked pop tarts on a parchment-lined baking sheet.
Brush the top of each pop tart with more egg wash and bake for 20-25 minutes or until golden brown.
Optional Glaze:

After baking, let the pop tarts cool slightly on a cooling rack. Drizzle with a simple sweet glaze made from powdered sugar and milk for an extra touch of sweetness!