Add flour, water, sourdough starter, honey, and sea salt to a bowl. Next using a knife, cut the cold butter into slices. Next, mix the dough with your hands until it begins to come together in large clumps.

Transfer the dough to a lightly floured work surface and fold the dough into itself until the flour is fully incorporated into the fats. The dough should come together easily and should not feel overly sticky. Avoid overworking the dough. If it feels too crumbly, dip your fingers in water and then continue bringing the dough together with your hands. If it feels too sticky, sprinkle more flour and then continue bringing dough together with your hands. Form it into a ball. Cover in a closed container or tightly in plastic wrap.

FIRST RISE: Ferment your dough (at room temperature) on your counter for 6-12 hours. Choose the coldest spot in your home! You can ferment overnight or during the day, whatever works best for your schedule.

SECOND RISE: Once the first rise is complete, place the dough tightly covered in the fridge overnight.

MAKE BLUEBERRY FILLING: Heat a medium saucepan over medium heat and add the blueberries and honey. Once the berries have softened to the point that they're starting to fall apart, add the chia seeds and stir them in well. Let the mixture come to a slow boil, stirring almost continually. Use a potato masher or fork to break up the blueberries. Continue stirring the mixture over low heat for about 10 minutes. Pour the jam into a large mason jar and allow it to cool, covered, at room temperature for half an hour before placing it in the fridge to cool completely (for 2 hours, or overnight). If you’d like to thicken your jam further, add in additional 1-2 tablespoons of chia seeds.

ROLL OUT DOUGH: Remove the cold dough from the refrigerator and allow it to sit at room temperature for 15 minutes. Slice the dough into two pieces so it's easier to roll out. Roll each into a rectangle about 1/8 inch thick. Trim the sides as needed. Cut each piece of dough into 6 rectangles. You will end up with 12 rectangles in total.

ADD FILLING: Place each of the 12 rectangles onto a baking sheet lined with parchment paper. Brush egg wash over the entire surface of each rectangle. These will be the bottoms of your poptarts and the egg wash will help glue the lid on. Place a heaping 1-2 TBSP of blueberry jam into the center of each rectangle and spread it around, leaving around 1/4 inch of space on the edges. Brush the second baking sheet of rectangles with egg wash, then place each rectangle on top of the filling-topped rectangles. Use your fingertips to press firmly around the pocket of the filling, sealing the dough well on all sides.

POKE HOLES & CHILL: Poke 8 holes on the tops of each filled pastry to allow the steam to escape. This helps get your pop-tart pastry nice and flaky. Seal the edges by crimping with a fork, to prevent the sides from opening as the pop-tarts bake. Refrigerate the filled pop-tarts uncovered for at least 30 minutes and up to 1 hour.

BAKE: Preheat oven to 350°F (177°C). Remove pop tarts from the refrigerator and brush the tops with the remaining egg wash. This egg wash will give your pastry that beautiful golden sheen. Bake for about 22-28 minutes or until they’re golden brown, rotating the pan halfway through baking. Let the baked pop-tarts cool on the pan for about 5 minutes, then transfer to a wire rack to cool completely before glazing.

MAKE THE GLAZE: Whisk all of the glaze ingredients together in a medium bowl until it reaches a spreading consistency. Add another teaspoon or two of milk if needed to thin the glaze. Use a spoon or knife to glaze each pop-tart. IMPORTANT NOTE: The glaze will not harden the same as traditional icing, so only glaze the poptarts when you are ready to eat!

STORING: Store un-glazed poptarts in an airtight container at room temperature for 3 days or in the refrigerator for 6 days. To reheat, bake in a 350°F (177°C) oven for 10 minutes and top with homemade glaze.