As of Hollow Knight's "Hidden Dreams" update, there is a small chance to acquire Essence when defeating an enemy. According to the wiki, this chance is quadrupled "if Essence was spent." However, as far as I can tell, the only way to spend Essence is by warping to a Dream Gate, which obviously removes you from any fight and is unlikely to put you in a new fight. How, then, does one spend Essence in such a way as to increase the odds of gaining Essence?
It just means, if you've ever used the Dreamgate, you're more likely to gain Essence until you've recovered all the Essence that was spent on Dreamgates. So you can't use this to farm Essence. (There's also absolutely no need to farm Essence, since the game has a lot more than you'll ever need, and even with the quadrupled rate it's a very very slow process to collect it from enemies.)