In a large bowl using a hand-held or stand mixer fitted with a paddle attachment, beat the butter on medium high speed until completely smooth and creamy, about 2 minutes. Add the confectioners’ sugar and orange zest, then beat until combined, about 1 minute. Next, add the egg, egg yolk, vanilla extract, and salt. Beat on high speed until combined, scraping down the sides and up the bottom of the bowl as needed. Finally, beat in the flour on low speed until combined. The cookie dough will be soft.