When Cas sees the brownies on the table, he can't help himself. Dean didn't expect him to find them, and what happens to be in those brownies cause all sorts of reactions to occur... (I'm sure you all know where this is going.)
Dean starts wearing the glasses used for seeing hellhounds around everywhere to discover other things hidden from his vision. At first he notices nothing. Until one day, Cas shows up and Dean can't seem to take them off.
Based off a post I saw on Insta about what if Dean could also see Cas' wings with the glasses on. Not my original idea:)