Well, why do you think they call it IMPOSSIBLE?!?!
All kidding aside, I'm sorry you are having difficulty with the recipe. Are you using the one from General Mills, or another? Maybe it has something to do with the mixing of the batter/dough...too long perhaps? Has the baking mix been used successfully in other recipes such that you can determine whether or not the mix is old/bad? I'm sorry I just asked more questions, but maybe if you sent a sample by air courier I could make a better determination.