I used to bake many bundt cakes when I was working; usually two at a time (one for my work group and one for DHs group). I used to nuke canned frosting and drizzle over cakes as a topping (didn't know the granulated sugar trick).
Yum. That looks really good. I have never heard of using butter with powdered sugar. I have used butter with brown sugar or turbinado sugar to dust the pan with. It's really good with banana nut bread.
I made pumpkin bread like that this winter season. DH took some to work. The guys asked how it was made said it was the best they ever ate.
When I do chocolate, I melt some butter. Stir in cocoa powder and use a pastry brush to coat the pan.....similar to @Nightowlz using butter and brown sugar.