Make the cookies: Preheat oven to 350 degrees. Beat butter, sugar, vanilla, and lemon zest and juice until combined. Beat in flour and 1/2 teaspoon salt. Wrap dough in plastic wrap, and refrigerate until firm, at least 2 hours.
Turn out dough onto a lightly floured surface, and roll out to a scant 1/4-inch thickness. Cut out about 18 cookies using 2 sizes of flower cookie cutters (one 3-inch and one 3 3/4-inch; fancyflours.com).
Transfer to parchment-lined baking sheets, and refrigerate for 30 minutes.
Bake until edges just start to brown, 14 to 18 minutes. Let cool completely.
Decorate the cookies: Transfer each batch of tinted icing to a separate pastry bag fitted with a small round tip (you can use as many colors as desired). Pipe an outline of icing around the edges, then "flood" each cookie with more icing to cover. Immediately
pipe a circle of small dots in the center of each cookie using a different-colored icing. Drag dots to outer edges using a toothpick to create thin lines. Repeat with remaining cookies and icing. Transfer cookies to parchment-lined baking sheets, and let dry completely,
about 30 minutes.
Once cookies are set, pipe a large dot in the center of each cookie using the same tinted icing as the small dots. Gently sprinkle poppy
seeds around large dot. Let dry completely.
Dough can be refrigerated for up to 3 days. Decorated cookies can be stored in an airtight container at room temperature for up to 3 days.