I very much like the Weyermann® Chocolate Wheat Malt Weyermann® Chocolate Wheat Matt
I use this in my Brown Ale. I'm a big fan of Weyermann's grain because my first preference in beer styles is German beer. I switched to this a couple of years ago from an English chocolate malt. For me, this works well.
1-2% of this malt in a porter will give you a head that will stay for days. It's a little lighter (about 100 SRM lighter) than barley-based chocolate malt so it won't add as much color as you'd think. But will definitely increase body and mouthfeel of your beer. It is out of style guidelines to make Porters with wheat but I think it's worth the try. When people ask you why the beer is so good you can just smile and call it "brewer's talent". Your secret will be safe here.