Overbeating the frosting will make it runny, so be sure to beat it just until blended, and make sure the butter and cream cheese are chilled before using.
Beat first 4 ingredients at high speed of a mixer about 2 minutes or just until smooth (do not overbeat). Gradually add powdered sugar, and beat at low speed just until blended (do not overbeat). Cover and chill 2 hours.
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