While cakes are still warm, in another large bowl, beat melted butter, 1 1/2 cups whiskey and the powdered sugar with whisk until smooth. In bowl, reserve 1 1/2 cups icing; set aside. With bamboo skewer, poke holes in tops of cakes. Pour some of remaining icing over cakes; let soak in 1 minute. Repeat until all icing has soaked into cakes. Cool completely, 45 to 60 minutes.