I was a very angry child growing up. I would explode over everything. I love my mom, but she never took the time to deal with it - it sounds like you are so that is awesome!! I beg you to continue the counseling. It may not have overnight effects but it may teach her life skills for coping with anger issues. I turned my anger on myself for fear of hurting anyone else - I would hit myself until I had enormous bruises or hit my head till the anger would clear. I would spend hours fighting myself and crying because I hated who I was. My mom said I kicked in a sliding glass door when I was three.
A couple of things to try: number one is switch counselors. Find someone anger and child geared. They have different training and focus on those problem areas.
Find help for the speech issues. A speech pathologist can help her find her voice! Make an appt. with her peditrician (a behavorial appt.) and they can refer you out to where you need to go. Many programs are free and will work anywhere from a daily to weekly basis. She may have a learning delay which can be helped and you will have a totally different child come home to you from these sessions.
Focus on her diet. Many people are trying gluten free diets and dye free diets with amazing results. Our son is autistic and we've started down many of these paths and have a much calmer and better behaved child.
Massage (touch) therapy. Also called qigong massage. It focuses on calming the nerves and creating a peaceful environment. I got a book when I was a teenager and performed some of the techniques on myself to help with my self-esteem (you might even like it! They have one to help clear cellulite and I swear it works!). We are training to do it on our little one. It's just a 15 min/day thing to do. You can talk to your peditrician again for a referral or call local therapy centers to find out if there's any training sessions near you.
A quiet area to calm down in. When she starts blowing up remove her from the situation and put her somewhere safe. I hurt myself a lot as a child. Get calming music (celtic was my fave as a teenager, but anything instrumental and calm will work). It will help her refocus her attention and simmer down.
Above all else try to remain calm. This is sooooo hard and it won't go away overnight. I didn't feel a break in my anger until late in my teens and still wrestle from time to time. I have awesome coping techniques and know how to calm myself down now. You will have to help teach her. She is a passionate individual - try to stay away from calling her an angry individual (she'll get that stuck in her head and it will take over - trust me on this one). You are really off to a great start even though I know it probably doesn't feel like it. When you need a breather TAKE ONE. It is so important that you get through this too. Best of luck and *hugs*. You can do it!