Preheat oven to 350 degrees. Line 3 baking sheets with parchment paper; set aside.
COOKIES: In a medium bowl, whisk together the flour, baking powder and salt; set aside.
In the bowl of a stand mixer using the whisk attachment, cream together the butter and sugar until light and fluffy, about 2-3 minutes. Add the egg and vanilla extract, lemon zest and lemon juice; beat for another minute.
With the mixer on low, gradually add the flour mixture; beat until all of the flour is incorporated, scraping the sides of the bowl as needed. The mixture will look crumbly at first; keep blending until the dough begins to pull away from the sides of the bowl. Using a toothpick, add a small amount of yellow gel food coloring if desired; beat until well combined, adding more until you reach desired shade of yellow.
Remove half of the dough from the bowl and form into a smooth ball. Roll the dough ball out onto a lightly floured work surface to 1/4-inch thick (do not make them thinner, you want the cookies to be on the thick side) and cut into 3-inch circles with a cookie or biscuit cutter. Repeat with remaining dough.
Place 8 cut-out cookies per baking sheet. Bake for 8-10 minutes or until the edges are slightly golden and the centers are just barely set.
Cool cookies on baking sheet for a minute or two, and then carefully remove them to a wire rack to cool completely before decorating
FROSTING: In the bowl of a stand mixer using a whisk attachment, beat butter, vanilla, milk, lemon juice and zest for 30 seconds. Add in confectioners’ sugar 1 cup at a time until you reach your desired consistency. (If frosting is a bit too thick add more milk a teaspoon at a time.)
Transfer frosting to a piping bag with a Wilton 2D drop flower tip. Begin piping in a tight circle in the middle of the cookie. Continue piping frosting in a circular motion until you reach the outside of the cookie. Sprinkle frosted cookies with yellow sparkling sugar.
Cook’s Note: Frosting will set, but don’t stack the cookies that have been frosted with a rosette or you will squish them. Cookies frosted with an offset spatula can be stacked once set.