You can substitute avocado for butter in a 1:1 ratio, but since it doesn't melt the same way it won't coat your dry ingredients the same. You need to increase your wet ingredients a little to compensate. In my kitchen, I just substitute avocados for half of the butter instead.
To do this, scoop out avocado, mash it in a small bowl and press it into a measuring cup. Combine with your softened butter and you're all set.
One more tip: try decreasing your oven temperature by 25 degrees (though is only really needed if you're substituting 100% of your butter) and watch your bake goods brown up as usual. They might have a slight green tint, but it's hardly noticeable in things with brown sugar.
It's a simple trick that can save you a trip to the store and some saturated fat. Plus, it's also delicious and that makes for a winner in our book!
(Image: Sarah Rae Trover)