Make the fruit preserves: add fruit, sugar, and lemon juice to a medium pot. Cook over medium heat until the sugar has dissolved. turn heat to high and boil 1-2 minutes, stirring occasionally. (optional: replace with Jam in recipe)
Preheat oven to 350°F. Line baking sheet with parchment paper.
Make the Dough: Mix the butter, sugar, powdered sugar, and vanilla until well combined. Add the sourdough discard and egg yolk and mix. Add Flour and mix on low until combined into a crumbly dough. Add water as needed until the dough comes together, creating a cohesive dough.
On a floured surface or parchment paper roll out the dough to a 14 inch square about 1/4 inch thick. (the thinner the better)
Trim the rough edges. Cut out 12 4 by 3 inch rectangles. Place 6 rectangles onto the baking sheet.
Make the Filling: Add fruit preserves or jam and cornstarch into bowl and mix together until well combined.
Assemble the pop-tarts: Spoon about 1 Tbs of filling onto the middle of each of the 6 rectangles. Dab water around the edges of each filled rectangle. Place the remaining rectangles on top. Using a fork, crimp the edges, then poke holes in the top of the dough.
Bake for 20 to 25 minutes. until the edges are lightly golden brown. Allow to cool completely before glazing.
Make the glaze: in a small bowl, mix powdered sugar, milk, and food coloring. Whisk together until a smooth, thick glaze forms. (If too runny, add 1 Tbs powdered sugar until desired thickness)
Frost each pop tart with glaze and top with decorative sprinkles.