If I use just enough flour to take ‘challah’ (with a bracha) can I use some of the dough for a yeast cake (mezonos). Does it make any difference if I intended to do this from the beginning or if I just decided to use some leftover dough after I had made my challahs.?
A dough that you plan on splitting, even if you plan on using the two parts of the dough yourself, if you would not want the two types of doughs to mix, then it is considered two separate doughs and you would not take off challah. Since you would not want the yeast cake to mix with the challah dough, because it will change its taste, therefore the two are considered separate dough’s. This does not automatically mean that you won’t take off challah. If one of the doughs are large enough to take challah even without a bracha, then you should take off challah from the whole dough, without a bracha, and then split it.
Y:D 322-2, Chazon Ish Zeraim (Likutim) 2 -2,3, Poskim.