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

Add the butter. Using a box grater, shred the very cold butter directly into the flour mixture. Toss gently so the shreds get coated in flour — this helps create a flaky crust without overworking the dough.

Add the sourdough & water. If using sourdough discard, stir it together with the cold water and add to the flour mixture. If you’re skipping discard, just add cold water a little at a time until the dough comes together.

Form the dough. Gently mix with your hands until it just forms a shaggy dough. Add a splash of extra cold water if it’s too dry. Don’t overwork it.

Chill. Divide into two disks, wrap, and refrigerate for at least 1 hour (up to 3 days).

Shape. Preheat the oven to 350°F (175°C). Roll out one disk on a floured surface to about ⅛-inch thick. Cut into heart shapes (or rectangles if you prefer).

Fill. Place half the shapes on a parchment-lined baking sheet and spoon about 1 tablespoon jam into the center of each.

Top & seal. Top with matching hearts. Press edges gently and seal with a fork. Chill on the baking sheet in the fridge for 20 minutes — this helps them hold their shape.

Egg wash. Whisk together the egg and heavy cream and brush over the tops. Use a toothpick to make a few tiny holes in each so steam can escape.

Bake. Bake for 25–30 minutes, until golden brown.

Cool & glaze. Let cool completely. Whisk together powdered sugar, jam, and heavy cream to make a glaze. Spoon over cooled pop tarts and add sprinkles if you like.