Flame out oak cubes or into boil kettle?

Has anyone done this before? I tried searching, but I haven't really found much. I boiled 1 oz for a 1.5 gal batch then I think I'm going to throw 0.5 oz into boil kettle and then I'm going to char the other 0.5 oz and throw it in after primary fermentation. At this will be for a 12% chocolate vanilla stout. Sounds kind of gross, but it's what my friend wants.