The Game of Doublets (formerly the "Revised New Game")

Vested

bested

Tested

Tester

pester

ester

Easter

aster

Faster

feaster

feasters

festers

fasters

This is one I don’t know what to do with – the official dictionary doesn’t allow ‘fasters’ (I tried it before my last word)… But I think it’s clearly a good word., I think we have to go by the official dictionary, though.

I thought I checked it, but you are correct. back to festers.

fester

fetter

Better

beater

Boater