1/4 cup whipping cream
1 tablespoon light corn syrup
4 ounces bittersweet (not unsweetened) or semisweet chocolate, chopped
1/2 teaspoon vanilla extract
Bring cream and corn syrup just to simmer in heavy small saucepan over medium heat. Remove from heat. Add chocolate and vanilla; whisk until melted and smooth.
Spread chocolate icing over half of each cupcake. Spread white icing on second half of each cupcake. Let stand until set, about 1 hour. (Can be made 1 day ahead. Store in single layer in airtight container at room temperature.)