Butter Avocado is a great substitute for butter in baking (measure to measure).  You certainly wouldn’t want to spread avocado on a baked potato.   
Buttermilk Use dairy or nondairy milk (measure to measure) with a tablespoon of either vinegar or lemon juice.  In some instances, plain yogurt can also be substituted (measure to measure).
Cocoa Powder Cacao powder (measure to measure), keep in mind that cacao is more bitter so you may need to increase the sweetener.
Lard/Shortening Coconut oil (measure for measure) 
Oil Unsweetened applesauce (measure to measure) can be used in place of oil in baking cakes and muffins.
Sour Cream Plain Greek yogurt