I just baked bialys for the first time, and the center of the bialys puffed up instead of retaining the flat center with the filling in the middle. Does anyone know the secret for maintaining the flat,crisp center? Before filling, I cut an "X" in  the center of the dough, but they still rose too high. Should less yeast be used? I have compared several recipes, and there is a difference of yeast amounts, using the same amount of flour, in different recipes.