In a large bowl, whisk together the flour, cornstarch and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, combine the butter and granulated sugar and mix until smooth and just beginning to lighten. Scrape down the bottom and sides of the bowl, add the maple syrup and mix to incorporate. Scrape down the sides and bottom of the bowl again, add the egg yolks one at a time and mix until incorporated. Add the vanilla and lemon zest and mix until just combined.
With the mixer on low, add the flour mixture and mix until just barely combined. Transfer the cookie dough to a sheet of plastic wrap and shape the dough into a disk while continuing to incorporate any flour. Cover and refrigerate for 20 minutes before rolling.
Preheat the oven to 350 degrees F. Line 2 baking sheets with parchment.
On a lightly floured work surface, roll out the dough to 1/4-inch thickness and cut into 3-inch flower shapes. Gather any scraps of dough together, wrap in plastic wrap and refrigerate for 10 minutes before rerolling and cutting shapes one additional time. Gently press a lollipop stick about halfway up each cookie, brush with a bit of milk and use scrap dough to secure the lollipop sticks. Flip the cookies over and place on the prepared baking sheets. Brush the tops with a thin, even coating of milk and sprinkle colored sanding sugar onto each cookie.
Bake until the edges of the cookies just begin to lightly brown, about 15 minutes. Allow to cool on the baking sheets for 5 minutes before transferring to a cooling rack to cool completely.