Sift the powdered sugar into a medium mixing bowl to remove any lumps. This critical step ensures your icing will have that signature smooth texture without any grainy bits.
In a microwave-safe container, melt the butter in 15-second intervals until completely liquid but not hot. The ideal temperature is just warm enough to be fully melted while maintaining the butter's flavor compounds – around 110°F (43°C).
Whisk together the melted butter, 3 tablespoons of milk, vanilla extract, and salt until well combined. The mixture should appear slightly separated at this stage – that's perfectly normal!
Gradually pour the wet ingredients into the powdered sugar while whisking continuously. This gradual incorporation prevents lumps from forming. Start with less milk and add more as needed – the perfect consistency should slowly drip from your spoon but not be runny.
Let the mixture rest for 2 minutes to allow the sugar to fully dissolve. After resting, assess the consistency – add a teaspoon of milk if too thick or a tablespoon of powdered sugar if too thin. Remember that the icing will thin slightly when spread on warm rolls.