1. Mix the ground flaxseed with the water and let it rest for 5 to 10 minutes until thickened.

2. Add the softened vegan butter, 1/2 cup granulated sugar, and brown sugar to a large mixing bowl.

3. Beat with an electric mixer for about 2 minutes, until light and fluffy.

4. Add the flax egg and vanilla, then mix for about 30 seconds.

5. Add the flour first, followed by the baking powder, pumpkin pie spice, and salt.

6. Continue mixing until a thick dough forms and holds together.

7. Cover the bowl and refrigerate the dough for 30 minutes to 1 hour.

8. While the dough chills, whisk together the pumpkin puree, maple syrup, oat milk, cornstarch, vanilla, and pumpkin pie spice until smooth.

9. Preheat the oven to 350°F and line a baking sheet with parchment paper.

10. Divide the chilled dough into 10 or 11 equal portions.

11. Roll each portion into a ball, then coat it in the reserved 2 tablespoons of granulated sugar.

12. Arrange the dough balls on the prepared baking sheet and press each one down slightly.

13. Use your thumb to create an indentation in the center of each cookie, then widen it carefully with your fingers.

14. Spoon about 1 1/2 tablespoons of pumpkin filling into each indentation, keeping the filling away from the outer edge.

15. Space the cookies at least 2 inches apart.

16. Bake for 12 to 14 minutes, until the edges are set and the pumpkin filling begins to firm and crinkle.

17. Let the cookies cool on the baking sheet for 5 minutes.

18. Transfer the cookies to a wire rack and allow them to cool completely.

19. Top with vegan whipped cream and a light sprinkle of pumpkin pie spice, if desired.