I did a peanut butter stout last year and used 2 containers of PB2. One at flameout and steeped the second container in the secondary. Overall head retention wasn't as bad as I had anticipated, but it was overall pretty light.
I didn't care for the batch, but many of my friends loved it. Some even blended it with shorts soft parade for a sort of "peanut butter / jelly" thing.
This is the recipe I used for a 5 gal extract batch. I'd imagine the lactose would just go near the end of the boil.
6lb LME
Grains (steeped for 20 minutes @ 158)
1lb Flaked Barley
1lb Carafa II
4 oz Black Patent
Hops
1oz Willamette @ 60min
Yeast
Danstar Nottingham - I made a starter 2 days prior
Could also use WL004 or WY1084
1 container PB2 @ flameout
1 container PB2 "dryhopped" for 5 days in secondary. I put the contents in a muslin sack to avoid adding floaties to the beer.
Added 1 dram of Lorann Peanut Butter Flavoring to secondary
Bottled with 4oz dextrose