Last night I drank the creme brulee stout and loved it for many reasons. I've made several milk stout myself and haven't gotten the thick heavy mouth feel that they have achieved. Would a higher mash temp do the trick or boiling longer? I love my stouts but they are a bit thinner in body.
The lactose has limited fermentability and adds body and mouthfeel. But I feel that an important addition is having 'enough' beta glucan in the wort. That is what contributes to thick and lasting heads and full mouthfeel. The problem is that is a haze former and quickly adds 'poor' flavor to the finished beer when in excess. The flavor aspect gets well masked in a stout or porter but shines through easily in paler beers.
Flaked barley is a big beta glucan contributor. Flaked wheat has about 10 times less. I suggest less than a half percent flaked barley in paler beers, but in a stout you can easily get away with several percent. I suggest not going overboard since you end up with a beer that produces a huge head and its a pain to pour. It looks nice though.
Foam Blowers of Indiana (FBI)