Step 1: Prepare the Dough - Mix flour, sugar, and salt in a bowl. Blend in cold butter until crumbly. Add sourdough discard and mix gently, adding cold water as needed. Shape into a ball, wrap in plastic, and chill for 30-45 minutes.
Step 2: Prepare the Filling - If using jam, thicken if necessary and let cool. Prepare other fillings as desired.
Step 3: Roll Out the Dough - Cut chilled dough in half, roll out one portion to ⅛ inch thick, and cut into rectangles.
Step 4: Fill and Seal the Pop Tarts - Place filling on half of the rectangles, brush edges with egg, top with another rectangle, and crimp edges.
Step 5: Chill Before Baking - Chill assembled pop tarts for 15 minutes and preheat oven to 375°F (190°C).
Step 6: Bake the Pop Tarts - Brush tops with egg and bake for 20-25 minutes until golden brown.
Step 7: Prepare the Glaze - Mix powdered sugar, milk, and vanilla until smooth. Drizzle over cooled pop tarts.
Step 8: Serve and Enjoy - Serve warm or store in an airtight container.
