Store in an airtight container in the refrigerator for up to 2 weeks. For a firmer consistency, keep it chilled; for easier spreading, let it sit at room temperature for 10 minutes.
Substitute half the butter with cream cheese for a cream cheese cinnamon butter variation.
Add a pinch of nutmeg or a dash of vanilla extract for extra depth of flavor.
Make sure to use confectioners (powdered) sugar, not granulated, to avoid a gritty texture.