It has long been a custom for Jews not to have bread made with milk. The reason is that you might forget. Half way through your two-day loaf, you might mistakenly use a couple of slices to make a salami sandwich. Bread, being a staple food, needs to be parev, able to be eaten with anything.
The Talmud (Pesachim 36a ) makes an exception for bread that is made k’eyn tora. There are a couple of explanations for this enigmatic phrase. Rashi writes that it means “like the eye of an ox”. In other words, if you are making a very small loaf, then it can be kneaded with milk. The logic is that it will be eaten in one sitting and not become leftovers to be eaten later when you may have forgotten that it is dairy.
The other interpretation of the phrase is that it means “in the shape of an ox” (Maimonides, Hilchot Ma’achalot Asurot 9:22). If the loaf is made cow-shaped, or has some other clear indication on the bread itself (eg a big letter M stamped on the side of the bread to show that it is made with milk — or meat — then, according to this view, the requirement for bread to be parev can be waived.