Just brew a porter as normal and add some cold-steeped (like overnight in the fridge) coffee at bottling time.
I've got a 5 gallon porter finishing up fermenting now. After it gets done cold-crashing, I plan on cold-brewing 1/4lb of course ground dark roast coffee (I may try Texas Pecan flavor on this batch), add about 2/3 of it to the bucket, taste it and then decide how much of the remainder I want to add.
Don't add coffee brewed with hot water unless you like the flavor of leftover burnt coffee that's been sitting in the heated carafe for several hours (or the taste of Guinness which, to me, is rather similar)
Good Temp Control -----> Happy Yeast ------> Tasty Brew