In the top of a double boiler, combine confectioners’ sugar, corn syrup, 2⁄3 cup (160 grams) water, and vanilla. Cook over simmering water, whisking constantly, until smooth. Add white chocolate chips; cook, stirring frequently, until chocolate is melted and mixture is smooth. Reduce heat to low.

Spoon 2 cups (661 grams) fondant into a liquid-measuring cup. Let cool until an instant-read thermometer registers 105°F (41°C) to 110°F (43°C); use immediately.