We don't know how the trend began, or moreover, why it even exists at all, but it does: "Bulletproof coffee," or coffee with butter added, is now a thing. Followers of the "Paleo Diet" are of the belief that eating like a caveman, ie: packing in large quantities animal fats and proteins, will boost energy, burn fat, and increase brain power.
If you want to to attempt the regimen (you glutton, you!) add a healthy tablespoon of unsalted, grass-fed butter, along with a coconut palm oil called MCT, to your daily cup of joe. The theory is that the added fat will provide a feeling of "fullness," but it's more likely that the added calories will lead to weight gain, and that the perceived energy boost is just a placebo.
There are very few things we wouldn't add to our coffee, but butter just might be where we draw the line. We have standards, you know.