Mix the dry ingredients: In a large bowl, whisk together flour, sugar, and salt.

Cut in the butter: Add cold butter cubes and use a pastry cutter or your fingertips to work them in until you have pea-sized bits with some larger flakes. Keep it chunky for flakiness.

Add starter and water: Stir in the sourdough discard. Sprinkle in cold water a tablespoon at a time, mixing gently until the dough just comes together. It should be slightly shaggy, not wet.

Chill the dough: Divide into two disks, wrap, and chill 45–60 minutes. This relaxes the gluten and keeps the butter cold.

Prep the filling: If your jam is runny, mix it with cornstarch and lemon juice. Set aside.

Roll out the dough: On a lightly floured surface, roll one disk into a rectangle about 1/8-inch thick. Trim the edges to square it off.

Cut rectangles: Cut the dough into 3 x 4-inch rectangles. You’ll need pairs for tops and bottoms. Gather scraps, chill briefly, and re-roll.

Fill the bottoms: Place half the rectangles on a parchment-lined sheet. Brush edges with egg wash. Spoon about 1 to 1 1/2 tablespoons of filling into the center, leaving a border.

Seal and crimp: Top with remaining rectangles. Press edges to seal, then crimp with a fork. Poke 3–4 small vents on top with a fork.

Chill again: Pop the tray into the fridge or freezer for 15–20 minutes. Cold pastry equals better layers.

Preheat and bake: Heat the oven to 375°F (190°C). Brush tops with egg wash, sprinkle with coarse sugar if you like, and bake 20–25 minutes, until golden and crisp.

Cool and glaze: Let the pop tarts cool 15–20 minutes. Whisk powdered sugar, milk, and vanilla into a thick but pourable glaze. Spoon or spread over the tops. Add sprinkles if you’re feeling festive.