Soften the butter: Let the butter sit at room temperature until it is soft enough to press easily with a spoon, but do not let it melt.
Combine the ingredients: Add the softened butter, packed brown sugar, ground cinnamon, and salt to a medium mixing bowl.
Start mixing: Beat the mixture on low speed until the brown sugar and cinnamon begin to blend into the butter.
Whip until creamy: Increase the mixer speed to medium and beat until the mixture looks smooth, creamy, and evenly combined.
Scrape the bowl: Use a rubber spatula to scrape the sides and bottom of the bowl so no brown sugar or cinnamon remains unmixed.
Check the flavor: Taste the butter and add a tiny amount of extra cinnamon if you prefer a stronger cinnamon flavor.
Serve or chill: Serve the butter soft with warm bread, biscuits, pancakes, or sweet potatoes, or transfer it to an airtight container and refrigerate.