This idiom or expression has to do with:

Making it more interesting.

We’re buttering the bread for our employees and giving them more benefits and vacation time.

Also:
Sweeten the deal.
Sweeten the lemonade.

Okay, I’ll sweeten the deal by giving you a free sixth night if you stay at our hotel for five nights in a row. How about that?!

Also see:
Up the ante.
Sweeten the pot.