BLACK AND WHITE SESAME SEED COOKIES

Steps:

• Cookies:
• Whisk baking powder, salt, and 3 cups cake flour in a small bowl to combine. Using an electric mixer on high speed, beat butter and sugar in a medium bowl until well combined (butter does not need to be fluffy), about 3 minutes. Add egg and egg yolk and beat just to combine. Reduce speed to low and gradually mix in dry ingredients. Divide dough in half and form into 3/4"-thick disks. Wrap in plastic and chill until firm, about 2 hours.
• Place racks in upper and lower thirds of oven and preheat to 325°F. Let 1 disk of dough sit at room temperature until slightly softened, about 5 minutes.
• Roll out dough on a lightly floured sheet of parchment paper, shifting often on sheet and dusting with more flour as needed to keep from sticking, until about 1/4" thick. Punch out rounds with cookie cutter and transfer to parchment-lined baking sheets, spacing 1 1/2" apart. Pat scraps into a 3/4"-thick disk. Chill 15 minutes.
• Bake cookies, rotating baking sheets halfway through, until edges are golden brown, 12-15 minutes. Transfer cookies to wire racks and let cool. Cut out and bake scraps, then repeat process with remaining disk of dough.
• Do Ahead
• Dough can be made 1 month ahead; freeze. Cookies can be baked 2 weeks ahead; wrap tightly and freeze. Thaw before decorating. Frosting and Assembly Toss black sesame seeds and oil in a medium shallow bowl to coat seeds (this will keep them jet-black and shiny). Place white sesame seeds in another medium shallow bowl.
• Frosting and assembly:
• Whisk powdered sugar, lemon juice, salt, and 1/3 cup water in a large bowl until smooth. Place half of the icing in a pastry bag fitted with a 1/8" tip. (Or use a large resealable plastic bag and cut a small opening in 1 corner.)
• Working with 1 cookie at a time, pipe down center and then around the edge of half of the cookie to create a semicircle. Pipe icing inside border to fill, then use a small offset spatula to evenly spread over half of the cookie. Working quickly so icing is still wet, dip frosted part of cookie into white sesame seeds, patting gently to adhere, then letting excess fall back into bowl. Let sit 15 minutes for icing to set.
• Repeat process on the other half of the cookie, dipping into black sesame seeds, and let sit at least 20 minutes before serving
• Do Ahead
• Cookies can be frosted and decorated 1 day ahead. Store airtight at room temperature.