Set a heatproof bowl over a pan of water; bring water just to a boil. Remove pan with bowl from heat.Advertisement
Put chocolate and cocoa butter in the bowl; stir just until melted. Remove bowl from pan; stir in corn syrup. Pour mixture onto a sheet of plastic wrap; fold sides in to cover. Let stand at room temperature overnight.
Knead white-chocolate mixture to soften, then knead it into rolled fondant until well incorporated (if it sticks, sprinkle work surface with cornstarch). Wrap in plastic; let rest overnight. Before using, microwave fondant on medium power for 5-second intervals until pliable.
If the dough starts to stick to the work surface while you're kneading, sprinkle the work surface lightly with confectioner's sugar or cornstarch. As with unflavored fondant, this recipe makes an ideal covering for a wedding cake, creating a smooth surface to use as a canvas or to present as is with a simple piped border.