Homemade Sourdough Pop Tarts with Jam Filling

Grate the butter and set aside. In a large bowl, whisk together the flour, sugar, and salt. Add the grated butter and mix until the dough resembles coarse crumbs. Stir in the sourdough discard and cold water.
Mix the dough with your hands until it all comes together. (Add up to 1/4 cup extra water if necessary to get the dough together).
Divide dough into two disks, wrap in plastic wrap, and refrigerate for at least 1 hour or up to 3 days.
When you're ready to bake, preheat oven to 350°F and line a baking sheet with parchment paper. Roll out one disk on a floured surface to about 1/8-inch thickness. Keep the other disk in the fridge until you're ready to roll it out.
Cut into 3x5-inch rectangles. Place half the rectangles on the prepared baking sheet. Add 1 tablespoon of jam to the center of each, allowing about 1/2 inch of space around the sides. Top with remaining rectangles, pressing down around the edges with your fingers and then a fork to seal.
Pop the baking sheets in the fridge (or freezer) to chill for 20 minutes. This helps prevent them from spreading too much.
Once they're done chilling, whisk the egg and heavy cream together for the egg wash, then brush over the tops of the pop tarts.
Bake for 25-30 minutes, or until golden brown. Let cool completely before you top with the glaze.
For the glaze, mix powdered sugar, jam, and heavy cream. Spread over cooled pop tarts and decorate with sprinkles.