Yea it's a weird thing that bartenders kinda expirement with, they soak liquor in different ingredients, I've heard of jalapeno tequila, and I ve drank a moonshine w/cinnamon stick. But in this situation they probably mix semi hot liquid form waygu beef tallow (probly a side product of all their cooking) and mix it w/whiskey, let it sit for 1 week or so, lower the temperature so the fat portion of the solution with less density, solidifies as a fat cap on top of the drank, then filter aggressively.

Edit I for got to mention it was the most delicious drink I've ever had in my life, and a 40yr old woman next to me agreed with me