1. In a small skillet, melt butter over medium heat. Butter will sizzle and crackle, then foam up. When it starts to smell toasty and you can see golden flecks on the bottom of the pan, take off the heat and scrape butter into a blender.
2. To blender add coffee, sweetener, rum, salt, and cinnamon. Blend on high speed until frothy, about 1 minute.
3. Pour into a mug and garnish with cinnamon.