I know this is old, but I was successful using actual peanut butter.
There's a trick to it.
Oil naturally separates from wort.
5 gallon keg batch.
Do a larger boil to end up with 6 gallons going into the fermenter. With 5-10 minutes left add the peanut butter. I used FOUR 1-lb jars.
You should have about 6.5-7 gallons of wort after adding the peanut butter.
Chill to pitching temp. Put the lid on it and let it sit for 20 minutes. The oil will separate out to the top.
I used my bottom drain to rack into the fermenter, but you could use a racking cane too. Ended up being about 5.75 gallons going into the fermenter. Try not to allow the top layer of oil to make it into the fermenter.
At kegging time, rack off the bottom only. Keep an eye on it as it gets close to the top layer of oil, stop the transfer. I ended up with about 4.75 gallons in the keg. The important part is to keep the oil out of the keg.
You can use any Robust Porter recipe you want but you need to add 1lb of Flaked Barley to it for head retention.
Here is my recipe;
12lbs Maris Otter
3lbs Chocolate
1.25lbs of Crystal 80
1lb Flaked Barley
2oz Northern Brewer 60min
.75oz Golding, East Kent 10 minutes
SAF-04 Dry Yeast
5-10 minutes left in boil add 4 Jars of Aldi Organic Peanut butter.
*This Peanut Butter just contains peanuts and sea salt (& it's $1.50 a Jar). I had put the jars into a pot of hot water when my boil started. The hot water was about 130F. By the time I needed to add the peanut butter to the boil it poured right out.
Great PB flavor and smell.