I'm making a mascarpone/whipped cream filling for some cream puffs. What's the right ratio of mascarpone to cream?

Don't ever whip marscapone by itself, always add Atleast a third cream or milk. If you whip the marscapone by itself, you'll turn it into a kind of gross cottage cheese:(

Excellent - thanks, everyone! I will not be doing any piping tonight (just a small dinner for my brother and SIL), but I will try the "whip them separately, the combine till it's right" method and report back.

You all rock!

I haven't mixed mascarpone and whipped cream, but when I mix creme fraiche and whipped cream, I do 1 to 1 -- it's fairly loose. So I'd start with 2 parts mascarpone to 1 part whipped cream, then add more whipped cream if needed.

Well, you have some room to play here. I'm no expert, but I would whip some cream and then whip some mascarpone--in separate bowls--perhaps a cup of each. Then begin by folding a little whipped cream into the mascarpone to lighten it. Fold in additional whipped cream and taste as you go until the consistency is as you would like it to be.

Are you using a pastry bag to fill the cream puffs? If so, you want to lighten the mascarpone enough by folding whipped cream into it that you can pipe the mixture into your pretty puffs. I don't know how many you are making, so I would suggest starting with half mascarpone and half whipped cream. Fold in the whipped cream gently, then see what the consistency feels like. Sounds luscious!