Set a heatproof bowl over a pan of water; bring water just to a boil. Remove pan with bowl from heat.
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.
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