Place 1/2 cup softened unsalted butter in a medium bowl. If the butter is not softened, leave it at room temperature until it is soft enough to press lightly with a finger.

Using a mixer, hand mixer, or a fork, beat or stir the butter until it becomes light and fluffy.

Add 1/4 cup brown sugar, 1/2 tsp ground cinnamon, and just a pinch of salt (omit the salt if you used salted butter). Mix thoroughly, scraping the sides and bottom of the bowl, until the mixture is smooth and fully combined.

Use the cinnamon brown sugar butter immediately, or transfer it to an airtight container. To shape for storage, form the butter into a log, wrap it in parchment, and place it in the container.

Refrigerate for up to 2 weeks.