Preheat the oven to 350F.

In a medium bowl, whisk together flour, salt, baking powder, baking soda, and cream of tartar. Set aside.

In a large bowl, cream together butter and sugar with a mixer.

Then, add in the flax egg and lemon juice. Mix until combined.

Gradually mix in the flour mixture, scraping down the bowl in between each addition. Then stir in the poppyseeds until incorporated.

Scoop dough into balls slightly larger than a tablespoon and place on a baking sheet lined with parchment paper.

Bake at 350F for 8-10 minutes, until cookies are set just at the edges. Allow to cool on the pan for 5 minutes and then move to a wire rack to finish cooling.