Whisk the brown sugar so that it is free of lumps. Remove any hard bits that could disrupt the texture of your frosting. Then lightly pack the sugar.r.
In a mixing bowl, add the butter, cream cheese, vanilla extract, and the brown sugar.
Use a mixer on medium speed to blend the ingredients for at least 2 minutes. Aim for a smooth consistency where the brown sugar has dissolved, eliminating any graininess.
Scrape down the sides of the bowl with a spatula to ensure all ingredients are incorporated evenly.
Gradually introduce the sifted powdered sugar to the mixture.
Start mixing on the lowest setting to prevent a powdered-sugar snowstorm. Once combined, increase the speed to medium-high and mix for another 30-45 seconds until the frosting is light and fluffy.
Taste the frosting and adjust the flavor or consistency as needed.