Lalvin EC-1118 Champagne
Had the same thing happen with a Cran grape + sugar ferment.
Put this yeast in? Plowed down from a stuck 1.020 to around 1.002.
Saved the batch. Plus kept fermenting as new CranGrape and CranRasp were added. Fast ferment.
1. Liquid yeast
2. Ferm temp <68F (live in hot climate)
3. Corny kegs
4. Recipes with >30% grain
5. More hop varieties available
6. More communication with brewers
Takes practice to avoid the over "over the top caramelly, malt and melanoiden laden beer" (huckdavidson above) and throw in the extra diacetyl from the malt. Difficult to get the flav just right with decoction.
What really helped mine was after 50F primary (FG < 25% OG), do a diacetyl rest for several days, at house temp (72F). Same container, no transfer.
That cleared out the butter bigtime.
Then transfer to serving keg.