In a large bowl, mix together 150 grams flour with sugar and salt.

Cut the cold, unsalted butter into small chunks and add to the flour mixture. Using a pastry cutter, cut the butter into the flour mixture until all of the butter is moistened by the flour and forms into a thick paste.

Add the remaining 175 grams of flour and use the pastry cutter or your fingers to distribute the flour until the mixture looks crumbly.

Pour the chilled sourdough discard on top of the flour/butter mixture and stir to combine. Use your hands to form the dough into a ball. If the dough is too crumbly, add ice cold water a Tablespoon at time until it comes together.

Cut the ball in half and form two balls of dough. Set one ball of dough to the side. Preheat the oven to 350ºF.

Flour the countertop and place the ball of dough on top of the flour. Roll one section of dough out into a 9 by 12-inch rectangle.

Use the bench scraper to mark the dough every three inches and every four inches. Use a pizza cutter to cut each rectangle into 9 equal-sized smaller rectangles.

Place 9 rectangles of dough on a parchment-lined baking sheet.

Whisk together egg with a splash of water. Brush egg wash on top of each rectangle of dough.

Spoon about 1 - 1 ½ Tablespoons jam into the center of each pop tart. Place a rectangle of dough on top of the jam filling, sandwiching the pop tart together. Crimp and seal the edges with a fork.

Bake at 350ºF for about 30 minutes until light brown, flaky and puffed up.

Let cool while you whisk together a glaze made of powdered sugar, milk, vanilla extract, and a pinch of salt. Spoon glaze on top of slightly cooled pop tarts and let cool before enjoying!