Black and White Sesame Seed 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.