Give your 15 foot snake to Goodwill, get at least a 25 foot snake.
A drain snake can drill right through a grease clog, the clog can just close up again when the snake pulls back out.
Sometimes giving the coil tip a very slight bend helps it gouge out a bigger hole in the clog.
If you snake properly, you should have hot water almost filling the drain line, and push and pull as you twist vigorously, it breaks up the clog better that way.
And the hot water can push the stuff you just drilled through out and away.
And your snake comes back out much less disgusting!
I would never pour boiling water down any drain or fixture.
Not safe to carry it, and bad for pipes, and can crack some fixtures.
Even when I drain pasta, I have cold water running down the drain first.
If snaking does not work, Instant Power from Home Depot has always worked well for me.
Buy a big bottle, half or full gallon.
Don’t get any on you, or anything else, and wear eye protection.
Let your sink stay unused for as long as possible, like 12 hours, then apply plenty of drain cleaner like a quart or half gallon, with a quart of hot water poured after it, then leave it sit for a half hour or an hour.
The hot water poured after pushes the drain cleaner out of the trap.
After a half hour to an hour, pour very hot but not boiling water into the drain, fill the sink up at least half way.
The weight of the water helps push the clog out.
Or hire a pro to snake better and farther in.
Or they can water jet the crud out.
Once you get it fixed, start using enzyme drain cleaner like Drain Defender, just a little bit once a week will gradually clean off lots of the crud that is lining your drain pipe like greasy jelly mixed with food and crud.