I have a lovely muffin recipe that works very well, and there is vegetable oil in it. I have been wondering if butter would work instead of oil, and if so, how much butter would I need instead of 125ml oil?
It is possible to replace oil with butter in a muffin recipe as generally most muffin recipes are quite forgiving and you may find that butter gives a better flavour. You can swap in 125g butter for the oil but the muffins may be a little dry and will possibly stale more quickly as the fat content of butter is lower than that of oil. For better results you may have to make some minor adjustments to the amount of fats and other liquids used. Butter is approximately 75% fat (the remainder being water and milk solids) so to switch butter for oil you need to increase the amount of butter used, but reduce slightly the other liquids (such as milk) used in the recipe.
For your recipe you will need to use just over 30% more butter, ie 170g butter, but you will need to reduce the other liquids by 40ml (170-125) to offset. Also you will need to melt the butter and let it cool slightly before using it. Switching butter for oil will work less well if the recipe has no other liquids added as the water introduced with the butter can toughen the glutens in the flour and the resulting cake will have a slightly different texture to one used with oil.
You can switch the other way (oil for butter) in muffin recipes where the butter is melted first, using 75% of the weight of the butter as oil instead and adding extra milk to make up the difference. However if the recipe requires the butter to be creamed first then oil will not work as this incorporates air and using oil will result in a flat cake or muffin.