In a mixing bowl, combine the flour, salt, and sugar. Mix thoroughly.

Add the cold cubed butter to the flour mixture. Use a pastry cutter or your fingers to blend until the mixture resembles coarse crumbs.

Stir in the sourdough starter until combined, forming a dough. Do not overmix.

Divide the dough into two equal parts, flatten each into a rectangle, wrap in plastic wrap, and refrigerate for at least 30 minutes.

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

Once chilled, remove one piece of dough from the fridge and roll it out on a lightly floured surface to a thickness of about ⅛ inch.

Cut out 6 rectangles (3x4 inches) from the rolled dough. Repeat with the second piece of dough.

On half of the rectangles, spoon about 1 tablespoon of fruit preserves in the center. Leave a border around the edges.

Brush the edges with the beaten egg and place a second rectangle over the top. Press around the edges to seal the tarts. Use a fork to crimp the edges.

Transfer the pop tarts to the prepared baking sheet. Brush the tops with the remaining egg wash.

Bake in the preheated oven for 25-30 minutes, or until golden brown.

While the pop tarts are baking, prepare the glaze by whisking together powdered sugar and milk until smooth.

Once baked, let the pop tarts cool for a few minutes, then drizzle with glaze and, if desired, add sprinkles on top.