Position a rack in the center of the oven and preheat to 350 degrees F. Line a baking sheet with parchment paper.
Combine the flour and salt in a medium bowl and whisk to combine. Put the butter, granulated sugar, confectioners' sugar and lemon zest in a large bowl. Beat with an electric hand mixer on medium speed until light and fluffy, about 3 minutes. Add the egg yolk and vanilla and beat until well combined. Add the flour mixture in 2 batches and mix until just combined.
Scoop the dough with a 2-ounce cookie scoop, filling it fully with the dough. Then shape it into a straight-sided patty (like a hockey puck) that is 2 inches wide and 1/2 inch thick. Put the formed cookie on the prepared baking sheet. Repeat to make 7 to 8 more cookies and making sure there is at least 1 inch of space around each cookie on the baking sheet.
Bake until the cookies have spread out and flattened by about half and the bottoms are lightly browned, 10 to 12 minutes, rotating the pan once halfway through the cooking time. Let cool 10 minutes on the baking sheet, then transfer to a wire cooling rack to cool completely.
Flip your cookies upside down so the flattest side is up. Use the yellow icing to make a thin line around the edge of each cookie. Then make 2 loops around the interior of the yellow border with the white icing and use a toothpick to fill in any gaps between the two colors. Fill in the middle space with the yellow icing and then make a circle with the white directly in the center. Immediately use a scribe or toothpick to lightly drag from the edge of the cookie to the middle to create the segments of your lemon. Repeat with the remaining cookies.
Let the icing dry and crust over or serve immediately.
When measuring flour, we spoon it into a dry measuring cup and level off excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)