Black and White Sesame Seed Cookies

Ingredients

• 3/4 teaspoon baking powder
• 3/4 teaspoon kosher salt
• 3 cups cake flour, plus more
• 1 1/4 cups (2 1/2 sticks) chilled unsalted butter, cut into pieces
• 1 cup granulated sugar
• 1 large egg
• 1 large egg yolk
• 1/2 cup black sesame seeds
• 1/4 teaspoon vegetable oil
• 1/2 cup white sesame seeds
• 4 cups powdered sugar
• 1 teaspoon fresh lemon juice
• Pinch of kosher salt
• A 3"-diameter cookie cutter

Steps

Cookies:

1. Whisk baking powder, salt and 3 cups cake flour together in a small bowl. In a medium bowl, beat butter and sugar with an electric mixer on high until well combined, about 3 minutes; the butter need not be fluffy. Beat in egg and egg yolk just to combine. Lower the speed and gradually mix in the dry ingredients. Halve the dough and form 3/4"-thick disks. Wrap in plastic and chill about 2 hours until firm.
2. Set racks in the oven's upper and lower thirds and preheat to 325°F. Leave 1 dough disk at room temperature until slightly softened, about 5 minutes.
3. On a lightly floured sheet of parchment, roll dough to about 1/4" thick, shifting it often and adding flour as needed to prevent sticking. Punch rounds with the cookie cutter and put on parchment-lined baking sheets 1 1/2" apart. Pat scraps into a 3/4"-thick disk and chill 15 minutes.
4. Bake for 12-15 minutes until the edges are golden brown, rotating the sheets halfway through. Cool cookies on wire racks. Cut out and bake the scraps, then repeat with the remaining dough disk.

Do Ahead

1. For advance preparation, make and freeze dough 1 month ahead, or bake cookies 2 weeks ahead, wrap tightly and freeze. Thaw before decorating. For frosting and assembly, toss black sesame seeds with oil in a medium shallow bowl to coat, keeping them jet-black and shiny. Put white sesame seeds in a separate medium shallow bowl.

Frosting and assembly:

1. In a large bowl, whisk powdered sugar, lemon juice, salt and 1/3 cup water until smooth. Put half the icing in a pastry bag with a 1/8" tip, or use a large resealable plastic bag with a small opening cut in 1 corner.
2. Work on 1 cookie at a time. Pipe down the centre, then around the edge of half the cookie to form a semicircle. Fill the border with piped icing and spread evenly over that half using a small offset spatula. While the icing is still wet, quickly dip the frosted half into white sesame seeds, pat gently to adhere and let excess seeds fall back into the bowl. Leave 15 minutes for the icing to set.
3. Repeat on the cookie's other half with black sesame seeds, then leave at least 20 minutes before serving.

Do Ahead

1. The cookies can be frosted and decorated 1 day ahead; store airtight at room temperature.