yeah, it was difficult for me as well until I found a way! If you pick the right choices that hints towards to Butterscotch's route, make sure that when coming towards the end of the play-through, Syrup got the crystal ball from Butterscotch and she is then later alone in the store, pick the top choice, "Look at the crystal ball" as the bottom choice would just lead you straight to Syrup's Ending. After that, you then click on another top choice, "Of course" when Toffee asks Syrup to come with her to see Treat(the bottom one would lead you straight to Butterscotch's Ending). By doing this, you will be able to get the Toffee ending.
Yeah, the funny thing is, right after I posted that, I went back to the game to give it one more go, and I got it immediately. All I needed to do to unlock Toffee was complain about them haha.