Preheat oven to 350 degrees F. In a small bowl, mix together ground flax seed and water; set aside. Combine dry ingredients in a bowl. In a medium bowl, cream together butter and sugar; add pumpkin puree, flax seed mixture, and vanilla, and mix well. Add dry ingredients. Scoop onto Silplat lined baking sheet (or use parchment paper). Bake 15-16 minutes. Let cool; gently use spatula to transfer cookies to a plate. Sprinkle with confectioner's sugar, if desired.