Monday marks National Coffee Day, and some java lovers are celebrating their passion (or addiction) by adding butter to their cup.

The drink, known as "Bulletproof Coffee," was made popular by the U.S. firm Bulletproof Executive.

According to the company's founder, technology entrepreneur Dave Asprey, Bulletproof Coffee can "jump start" your body with the "right fats," which in turn can help boost your energy.

Asprey also says the special coffee will not induce "crashes" in energy levels, commonly experienced with "regular" coffee.

He said by drinking Bulletproof Coffee in the morning, and following other steps in his "Bulletproof diet," he was able to lose 100 pounds and improve his IQ.

"It will keep you satisfied with level energy for six hours if you need it, and because I'm having it for breakfast, I'm programming my body to burn fat for energy all day long," he says on the company's website.

"This exact recipe works every day, not just for me, but for chart-topping musicians, professional poker players, MMA fights, Olympic athletes, entrepreneurs, and Fortune 500 CEOs."

To make the special coffee you'll need to use filtered water, "Bulletproof Upgraded" coffee beans, an oil extract called "Brain Octane Oil" – which contains medium-chain triglycerides (MCTs) that the company advertises as being 18 times stronger than regular coconut oil-- and butter coming from grass-fed cows.

Asprey notes that it is especially important that the butter be the right kind. "Don't mess with cheap butter," he warns in a video showing how to brew Bulletproof Coffee.

But some critics are warning that drinking coffee laced with butter and MCTs may be problematic.

Popular nutrition researcher Kris Gunnars says that by consuming a cup of coffee brewed with butter and MCTs instead of eating breakfast, you're "effectively replacing" one of three nutritious meals with something that is low in essential nutrients.

"Yes, grass-fed butter contains some fat-soluble vitames (A and K2), CLA and butyrate," he wrote in a blog post. "But MCT oil is 100% empty calories. It is a refined and processed fat with no essential nutrients.

"Even though Bulletproof Coffee may contain small amounts of nutrients, this completely pales in comparison to what you would get from a nutritious breakfast."

Gunnars also said that fans of the special brew should consider the amount of saturated fat going into the coffee and the potential risk of increasing your cholesterol levels as a result.

Nutrition consulting company Precision Nutrition also raises concerns about excessively drinking Bulletproof Coffee. It estimates that a cup of Bulletproof Coffee contains 10 times more calories than a standard cup of coffee with cream.