For new pipesmokers, getting cake is the first physical benchmark. A lot of importance is placed on that cake. I get that. But, really once you get a nice carbon layer, you spend the rest of your days with a wall full of pipes, carving that crap back to a manageable layer.
It's not that big a deal. You will think that it will make every bowl smoked from there on out magical. Better, maybe, but not exactly magical. So...
Anyways, I just smoke whatever tobacco I would normally smoke in the pipe. I enjoy getting hints of the briar in the first few bowls. It gives me an idea of what the briar aroma is going to be there on out.
But really, just relax, roll with it, don't hurry, and enjoy the whole break in process. Don't be in a hurry to reach that cake benchmark.