I found a yellow cake recipe that I like. It tastes great on the first day but it tends to get really dry on the second day. The recipe calls for all 1 cup of butter, no oil. I'd like to add oil to this recipe to see if it helps to keep it moist. Do I just add the oil or do I take out something else to account for the additional oil?
I would just start replacing equal parts of the butter for the oil.
I would try half a cup each of oil and butter...
Butter is only 80% fat. If you start adding equal parts of oil, you will be substituting too much oil and leaving out the extra liquid that comes from the butter.
Lets say it is 8 ounces of butter, times .80 is 6.40 ounces of fat and the rest if liquid. So for every 8 ounces it is 1.6 ounces of liquid.
If you want to substitute 4 ounces of butter for oil divide by .80 and then multiply the liquid by 1.20 to get the amount of extra liquid to add back in.