Whisk flour, cornstarch, salt, baking powder and baking soda together in a medium bowl. Set aside.
Combine butter and sugar together in the bowl of a stand mixer. Mix on medium speed until light and fluffy, 2-3 minutes, scraping down the sides of the bowl as needed. Add lemon juice, egg yolk and lemon extract and mix until fully combined. Reduce speed to low and slowly add dry ingredients until dough just comes together.
Wrap the dough with plastic wrap and press into a 6 inch disk. Refrigerate until firm, at least 1 hour or up to 1 day.
Preheat the oven to 325 degrees and line a baking sheet with parchment paper. Remove the dough round from the fridge. If it has been in there awhile, it may be really firm and need to warm up slightly before you can start rolling. Divide the dough into 2 pieces and dust a clean work surface with flour.
Working with one piece of dough at a time, dust the top of the dough with flour and roll with a rolling pin until it is between ¼ and ⅛ inch thick. Add more flour as necessary to prevent the dough from sticking to the counter or the rolling pin. Cut round shapes using a 2-2 ¼ inch cookie cutter, preferably one with a scalloped edge. Carefully transfer the unbaked cookies from the counter top to the prepared cookie sheet.
Bake for 11-13 minutes or until cookies are almost beginning to brown on the edges. Cool slightly on the baking sheet and then transfer to a wire baking rack to cool completely. Repeat with remaining dough, gathering and re-rolling scraps as needed.
Once cookies have cooled to room temperature, whisk glaze ingredients together. Using a small spoon, top each cookie with a tiny dollop of glaze and spread with the back of the spoon. Allow glaze to dry before serving, at least 15 minutes.