In a medium saucepan over medium heat, combine butter, brown sugar, and heavy cream.
Bring to a boil, stirring regularly. It's fine to stir occasionally as the mixture is melting and heating up, but be sure to stir constantly as it begins to simmer, and as it turns to a rolling boil.
Once it reaches a boil, continuing cooking for 1-2 minutes, or until sugar is dissolved. When it loses it's grittiness, and sugar granules are no longer visible, remove from heat.
After removing from heat, whisk in vanilla extract.
Cool mixture to room temperature.
Transfer the caramel mixture to a large mixing bowl. Fit your mixer with a whisk attachment.
With the mixer set to low speed, gradually add 4 cups powdered sugar.
Beat on medium speed for 2 minutes.
Add additional powdered sugar, as needed, to achieve desired consistency, and beat for 1 minute on medium speed before settling on consistency.
Frost anything!
Store in a tightly-sealed container for 3-4 days at room temperature, up to 2 weeks in the refrigerator, and up to 3 months in the freezer.