In the bowl of a hand or stand mixer fitted with the paddle attachment add the 1 cup butter and mix on low-medium speed for 30 seconds-1 minute.
Stop the mixer, add in the 3 cups of powdered sugar, and turn the dial to the lowest mixing setting so powdered sugar doesn't blow up in your face. Move to a medium speed when the sugar is more incorporated. Mix for 2 minutes.
Stop the mixer and add in the 1 tsp vanilla extract and 2 tsp cream. Mix on medium-high speed for 1 minute-2 minutes.
The icing should have stiff peaks, and hold its shape, not runny at all.