1 cup powdered sugar
2 tablespoons whipping cream
2 teaspoons fresh lemon juice
Whisk all ingredients in small bowl to blend. Let both icings stand 1 1/2 hours at room temperature to thicken.
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.)