Assuming you are using plain unsweetened yogurt, be advised that even after you drain/strain it, when you add sugar to it, it will thin out again somewhat. There's really not a lot of ways you can thicken it up to the point where it has the consistency of buttercream if that's what you're after. Some Southern buttercreams are thickened with a roux somewhat and I suppose you could try that, but it sounds kind of terrible to me. I don't think you could stiffen it with whipped butter either, as it would most likely break if it would even come together at all. If it were me, and I wanted to use yogurt, I wouldn't try to make the yogurt something it's not......I'd use it as a glaze type of icing if anything. Mix in a little powdered sugar, whatever other flavorings you like, and thin it down with a little milk or lemon juice depending on how tart you want it. Then drizzle it over your muffin tops.
