In a large mixing bowl, whisk together the flour, sugar, and salt to create your flour mixture.

Add the grated chilled butter and mix with a pastry cutter until the mixture resembles large crumbles.

Add the sourdough starter discard and cold water. Mix until the pastry dough comes together. Avoid over mixing.

Transfer the dough to a lightly floured work surface and gently knead it into a disc shape.

Divide the dough in half, wrap each half in plastic wrap, and refrigerate them for at least 1-2 hours. This dough chill time is essential for developing those flaky layers!

Preheat the oven to 375°F (190°C) and line a baking tray with parchment paper.

On a floured work surface, use a rolling pin to roll out one half of the dough into a large rectangle about 1/4-inch thick. Using a bench scraper will help to lift and move the dough if it sticks.

Using a pizza cutter or sharp knife cut the dough into long rectangular shapes, approximately 3 x 8 inches. Repeat with the second half of the dough and any scraps.

Using a pastry brush, brush the edges of the dough with egg wash.

Spoon 1 tablespoon of jam onto the center of each brushed rectangle, leaving a 1/3-inch border around the edges.

Place the unfilled side of the dough rectangles over the jam-filled side.

Press down gently around the edges to seal, then crimp the edges with a fork.

Brush the top of each pop-tart with the egg wash for a golden, glossy finish.

Use a toothpick or fork to poke small holes on the top of the pop-tart to allow steam to escape during baking.

Place the unbaked pop-tarts on the prepared baking sheet and bake for 15-20 minutes (or 20-25 minutes for larger tarts), or until golden brown on top and slightly darker on the bottom.

Remove them from the oven and let them cool slightly before glazing.

In a medium bowl, whisk together the milk, vanilla extract, and powdered sugar until smooth. If the glaze is too thick, add more milk, 1 teaspoon at a time, until it reaches a pourable consistency.

Spread or drizzle the glaze over the slightly cooled pop-tarts.

Optional: Sprinkle with decorations like colored sugar or sprinkles. Let the glaze set for a few minutes, then enjoy!