Fat washing is mixing a melted fat with a spirit, chilling the mixture until the fat solidifies, then skimming it / straining it, to get the fat out. I use a cheesecloth or even a coffee filter sometimes.
No fat is left in the spirit BUT the taste stays. When done properly, the only thing left behind is the taste of the fat (the spirit itself does not taste fatty or greasy). Sounds weird, tastes delicious.
So far I've tried it with bacon fat (for bacon-infused bourbon) using the Don Lee/PDT recipe. It leaves a subtle smokiness behind, dependent upon how smoky the fat was to begin with. No actual bacon pieces are required. Just the fat.
And I've been messing around with browned butter (for butter-infused rum) using Eben Freeman of Tailor's recipe. Makes a mean hot toddy, and finishes quite nicely. However, I've found that Freeman's recipe calls for a LOT of butter per cup of rum, and my infusing vessel doesn't appear to be making the best use of the butter as it is not superwide.
Anybody else here infusing spirits using fat washing? What fats besides bacon fat or butter? I have been told that sesame oil does not work too well, neither does olive oil, or avocado.