Frosted Valentine Cookies

To make the cookie dough:

1. In a large bowl, sift together flour, baking powder, and salt. Set aside.
2. In a stand mixer with the paddle attachment on medium speed, or with an electric mixer, beat butter and sugar until pale and fluffy. Beat in egg yolks and vanilla.
3. Reduce mixer speed to low and gradually add flour mixture and milk until just incorporated. Scrape dough on to a floured work surface, shape into a ball, and wrap in plastic wrap. Chill at least 2 hours or up to 2 days in advance.

To bake the cookies:

1. Preheat oven to 350 degrees. Line 2 baking sheets with parchment paper or silicone mats.
2. On a floured surface, roll chilled dough between parchment paper or wax paper to 1/8-inch thickness. Cut with heart-shaped cookie cutters. Transfer to prepared baking sheets with an offset spatula. 
3. Repeat with the remaining dough. Scrape all remnants together and re-roll. If the dough becomes too soft or sticky, refrigerate an additional 10 minutes.
4. Bake until the edges are lightly browned, about 8 to 10 minutes (if thicker than 1/8 inch, they may take 10 to 15 minutes). Transfer to wire racks and cool completely before decorating with frosting (recipe follows).

To make the royal icing:

1. In a stand mixer fit with the whisk attachment on medium-high speed, add the egg whites and vanilla and beat until foamy.
2. Reduce mixer speed to low and add the powdered sugar until just incorporated and shiny. Beat on high for 5 to 7 minutes or until the mixture is thick and glossy with stiff peaks.
3. Add food coloring if desired, then transfer to piping bags or spread with a spatula. If using sprinkles or sugar, add quickly before the frosting hardens.
4. To make in advance, store in an airtight container in the refrigerator for up to 3 days.