Make the Dough:
In a large mixing bowl, combine flour, sugar, and salt.
Grate or cut in cold butter until the mixture has a coarse, crumbly texture.
Add sourdough discard, cold water, and vinegar. Mix until a rough dough forms.
Divide dough into two discs, wrap in plastic, and chill for 1-2 hours.
Prepare the Filling:
For a jam filling, mix the jam with cornstarch for a thicker consistency, if desired.
You can also use fillings like Nutella, a brown sugar cinnamon mix, or lemon curd for variety.
Roll and Cut the Dough:
Preheat oven to 350°F (177°C) and line a baking sheet with parchment paper.
Roll out the dough on a floured surface to about 3mm thickness.
Cut into 12 rectangles, each approximately 8cm x 12cm.
Assemble the Pop Tarts:
Place 1-2 tbsp of filling in the center of 6 dough rectangles, leaving the edges clear.
Top with the remaining 6 rectangles, pressing the edges with a fork to seal.
Poke small holes in the tops and, if desired, brush with an egg wash.
Bake:
Bake in the preheated oven for 25-30 minutes, or until golden brown.
Let cool on a rack for about 10 minutes.
Make the Glaze:
In a small bowl, whisk together powdered sugar and milk until smooth. Add vanilla extract if desired.
Once pop tarts have cooled, drizzle or spread the glaze on top and allow it to set for about 1 hour.