Prepare the blueberry chia filling by placing the ingredients (minus the chia seeds) into a small pot over medium heat. Cook the filling until the blueberries begin to release juices. Use the back of a wooden spoon to smash the blueberries as much as you can (remove 1 teaspoon of juice and set it aside to use for the glaze). Simmer for 5 minutes and then turn off the heat. Finally stir in the chia seeds and then let your jam cool.
Make the crust by placing the flour, sugar and salt into into a food processor and pulse until combined. Cut your butter into cubes and them to the flour mixture and then pulse until the butter is in small chunks and evenly distributed. Add in the water one tablespoon at a time, pulsing between each addition until the dough forms crumbles.
Pour the crumbles onto a clean work surface and press them into a mound. Knead the dough until smooth, about 3-5 turns. Separate the dough into 2 evenly sized pieces.
Preheat your oven to 375 F° and prepare your work surface with parchment paper dusted with flour. Roll out each piece of dough into rectangles about ⅛ inch in thickness. In one of the rectangles, use a fork or toothpick to poke holes so air can escape while baking.
Add the filling to the bottom dough leaving about 1 inch of room around the edges. You will have a bit left over, save this for the glaze and future use. Top with the second dough and seal them together using a fork to crimp the edges. Brush the entire pastry with egg wash.
Place the pop tart on a baking sheet and bake for 30-40 minutes and golden brown. Let cool before stirring together the glaze and pour over the pop tart. Finish with sprinkles if desired. Cut into 12 pieces and serve.