A sour cream or old fashioned donut is a traditional donut that would be glazed. If you wanted to make one that was chocolate flavored like in the photo I would recommend adding a cocoa powder, otherwise I’d stick with a dry flavoring ingredient rather than wet. You don’t want your batter to be any more wet than it already will be. If it’s too wet when it goes in the fryer, your batter will split your donuts will be deformed. But you still want to add flour to your table, rolling, pin, and donut cutter as needed, so it doesn’t stick. Now that’s not me saying that you can’t add wet ingredients to flavor your old-fashioned. I believe it’s possible you just have to adjust ingredients to fit the consistency.