What Chocolate in Porter

I brewed a Porter recently, it is OK but not memorable. I fancy making a Chocolate Porter with chocolate in it. Any recommendations or tips on what to use. I believe you can use Cocoa Powder, but how much and when would you add it. I have also seen mentioned Cocoa Nibs What are they? Are they better than powder? Where would you get them?

Malt Miller has cocoa nibs.

I've got 100g of cacao nibs that I'm going to dry nib a porter with this week. Just looking at the best way to do it, here's what I'm planning in doing unless anyone has any better options:

Roast nibs at 150c until there's some chocolate aroma coming off them. Crush lightly and soak in vodka for 24 hours, throw it all into primary (primary fermentation has stopped) and sample daily until ready to rack into a keg.

I am planning to add nibs to a porter soon. Reading round I was going to soak them in vodka and add them to secondary. Some advised against adding to primary because they can become quite bitter.