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.
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.