Ultimate Sugar Cookies

Place racks in lower and upper thirds of oven; preheat to 325 °F. Whisk salt, baking powder, and 3 cups flour in a small bowl. Using an electric mixer on high speed, beat butter and sugar until well combined (butter does not need to be fluffy), about 3 minutes. Add egg, egg yolk, and vanilla; beat just to combine. Reduce speed to low and gradually add dry ingredients; mix just to combine. Form dough into two 3/4"-thick disks; wrap in plastic. Chill at least 2 hours. Let 1 disk of dough sit at room temperature until softened slightly, about 5 minutes. Roll out on a lightly floured sheet of parchment paper until about 1/4" thick, dusting with flour as needed (if dough gets soft or sticky, chill on parchment until firm). Cut out shapes with floured cookie cutters; transfer to parchment-lined baking sheets. Bake cookies, rotating baking sheets halfway through, until edges are golden, 12 –16 minutes, depending on size. Transfer to wire racks and let cool. Repeat with scraps and remaining dough. Decorate cooled cookies as desired. MAKE AHEAD: Cookie dough can be made 1 month ahead; freeze instead of chilling. Cookies can be baked (left undecorated) 2 weeks ahead; wrap tightly and freeze.