Add the butter and cream to the bowl of your stand mixer. Mix on medium speed with the whisk attachment, until the butter and cream combine.
Increase the speed to high and mix for 3 to5 minutes, until the mixture is light and fluffy, stopping to scrape down the sides a time or two.
Add the vanilla and 2.5 cups of the powdered sugar. Mix on low until combined, and then increase speed to medium and whip for 1 to 2 minutes.
The consistency of the icing is important if you are going to ice sugar cookies with it. It should be a soft-medium consistency and firm, but soft enough to spread with a butter knife.
If the icing is too thick, mix in a little cream. If it needs to be a little thicker, add a 1/4 cup of powdered sugar, and whip to combine until it’s sturdy enough.
Use icing to frost around 2 dozen medium sugar cookies. It also pipes well, if you'd like to put it in piping bags with tips.
Notes
This is a great recipe to add food coloring to if you'd like to decorate seasonal sugar cookies. I'll often make a batch, divide it into smaller bowls, and dye each bowl colors.
You are going to add sprinkles to the cookies, do so just after spreading the frosting. They'll stick better that way. If you wait until the frosting has time to set, it forms a bit of a crust, so sprinkles won't stick as well.
If you are layering colors of frosting, like when using a piping bag, let the first color dry (or crust) before applying the second color so they don't bleed.
Consistency is super important when frosting cookies. You might need to thin it with a little more cream or thicken it with a little more powdered sugar.