How long have you been training? Begginers can gain muscle while losing fat eaisly up to a point. After that your progress will stall if you don't make the decision to do one or the other.
You can be lean while gaining muscle as long as your diet is in check. As long as you are taking in a surplus of calories you will not lose muscle as long as you continue to lift with the same or greater intensity. You may gain some fat in the process. How much fat you gain is a function of how much you eat and how quickly you want to put on muscle. It is very hard (i.e. takes a very long time) to gain a lot of muscle without putting on some fat.
If you want to be a powerlifter you will need to eat and lift hard without worrying too much about body fat. Pick a goal and stick to the plan that will get you there. If it is really important to be at 10-12% body fat, then diet until you get there. You will lose some muscle in the process since you are not that big to begin with. Once you are there, then pick a goal and go balls-out to get there (i.e. weigh 200+ lbs and bench 100+ more lbs or whatever). You will gain some fat back in the process, but it will be much easier at that point to cut to remove it, due to your increased muscle.
Personally, I don't think 15% is that high - I would start the bulk now without looking back.