I have a coconut pie recipe that uses 1/2 cup butter and 3 eggs. Could I replace some of the butter with condensed milk? Any other substitutes besides margarine?
Butter is about 80% fat, maybe a bit lower if you're getting it direct from the farmer; condensed milk is about 10% fat, so you'd need to use a lot of condensed milk and you'd probably have too much excess water to compensate by reducing extra liquids.
The best substitute would be another fat which is solid at room temperature. There aren't many (at least, not if you restrict to vegetable fats), but one which does meet the criterion is coconut oil, and since you're making a coconut pie that seems like an excellent substitute.
Honestly, butter is somehow far away from condensed milk so I don't think you can use condensed milk as a substitution of butter at all. your pie will turn out totally different. As opposed to butter, you can use oil instead such as vegetable oil or coconut oil, both will work well. hope it helps. cheers