Sift confectioners' sugar several times into a bowl to eliminate all lumps.
Beat butter, corn syrup, and salt with an electric mixer on medium speed until light and fluffy, about 5 minutes. Beat in the sifted confectioners' sugar until thoroughly incorporated.
Turn fondant out onto a work surface generously dusted with confectioners' sugar. Knead until smooth and workable, 5 minutes; cut dough in half. Wrap each piece of dough in plastic wrap and chill for at least 30 minutes.
