I have seen cookiers make a thinned down royal icing and paint a thin layer on the cookie. They use a pastry or silicone brush. Here's a quick youtube video I found on it called "brush flooding". I would make the white icing a thin layer so you can still see your outline for the rest of the details. I have that cutter too. GO CUBS GO!!!! https://www.youtube.com/watch?v=MAloikWLErk