Actually, I know exactly when they went wrong - I just have no idea why. I started by creaming 5 tbsp of butter and 1 cup of sugar until fluffy, then added two eggs and beat those in. At this point the batter was fluffy and creamy and smooth. The next step called for the addition of 2/3 cup of milk. I only have 1% milk in the fridge so I added 2/3 of a cup of that and started to mix... only to find that my previously smooth batter had separated into small white chunks of stuff floating in a cloudy yellowish liquid. The batter tastes exactly like batter at this point should, and none of my ingredients are spoiled.

Does anyone know what happened there? Could it have been because I used 1% milk instead of whole? Thanks!