Yosef Karo, Shulchan Aruch — Yoreh Deah 97
“ One may not knead dough with milk, lest one come to eat it with meat. If one did knead it that way, the whole loaf is prohibited even to eat alone. If it was a small quantity that can be eaten in one sitting, or its shape is unusual so it will be recognizable that it shouldn't be eaten with meat, it is permitted. One may not bake bread in an oven that was smeared with fat, and if one did, the rules are as for a dough kneaded with milk. ”
