In a large bowl, whisk together flour and sugar.
Add in the chilled, cubed vegan butter. Cut the butter into flour using a pastry cutter or two butter knives until mixture becomes crumbly and resembles a coarse meal.
Stir in the ice water, one tbsp at a time, until the dough just comes together to form a ball. I usually add about 5 tbsp of ice water. Then, gently form dough into a large disc, divide in half, and wrap each half separately in plastic wrap. Refrigerate dough for at least 30 minutes or up to 24 hours.
Preheat oven to 375F. Once chilled, remove one half of dough from fridge and let sit for 10 minutes at room temperature. On a clean and floured surface, roll the dough into a large rectangle, about 1/8th inch thick. Trim edges to create a clean rectangle. You can re-roll any scrap pieces of dough as needed at the end.
Cut the clean rectangle into evenly sized smaller rectangles, about 3x4 inches each.
Use a fork to prick the centers of half the small rectangles. These will be your tops.
Place 1-2 tbsp of the chia jam in the center of the remaining rectangles. Place the pricked rectangles over the jam, and use a wet fork to crimp and seal the edges.
Place poptarts on a parchment lined baking sheet, and place in fridge while you repeat the process with the second half of the dough. If desired, brush or spray the tops of the poptarts with oil or nondairy milk to get golden brown colored poptarts. Once all poptarts are prepared, bake at 375F for 25-30 minutes.