Beat butter and sugar together until light.
Add flax mixture and vanilla and continue beating until light and well mixed.
In a medium bowl, whisk together both flours, salt, cinnamon and lemon zest.
Add flour mix to wet mix and blend together, but don't overmix.
Divide dough into two parts, wrap in cling wrap and chill for at least an hour.
Preheat oven to 350 degrees F. Line a baking sheet with parchment paper.
Remove one of the chilled doughs from the fridge and sprinkle a bit of flour on a sheet of parchment paper. Add dough to the center of the parchment then top with the second sheet of parchment paper. Roll to 1/8- 1/4 inch thickness. Cut out round or heart shapes with floured cookie cutters and then, using a floured spatula, gently move the cutouts to the prepared pan. Reroll scraps until all dough is used. (*1)
Bake for approximately 10-14 minutes. They should be lightly brown just around the edges.
Remove 2nd chilled dough and roll it out to 1/8- 1/4 inch thickness. Cut out with the same cutter and process used in step 7 above. I transfer them to a parchment-lined cookie sheet now. Then using a small cookie cutter, cut out a circle, heart, or another small shape from the center of each 2-inch cutout cookie. If the tiny cutout doesn't come out easily with the cookie cutter, just use the tip of a bread knife to gently lift the little cutout off the sheet and set it aside. This helps keep the shape without any distortion that can happen while moving to the cookie sheet. Reroll scraps until all dough is used, making sure there is an equal amount of cookie tops and cookie bottoms. Baking time may be slightly less for the top cookies than for the bottom cookies.
Cool cookies on a wire rack. When completely cool, dust powdered sugar on only the tops of the cookies with the cutout centers.
Stir jam until smooth. Flip the cookies without the cutout centers over to their bottom side. Spread about 1/2 -1 teaspoon on each bottom. Then top with the sugar-dusted cookies. (*2)
Store in an air-tight container at room temperature for up to about 5 days.