Place the butter in a small ramekin or glass bowl and microwave on high for 10 seconds. Check it by stirring with a spoon to see if it is the consistency that you want: opaque and creamy, thin but not clear, like melted chocolate. If it is not there, repeat for 10 seconds more, and then check again. Repeat until the butter is barely melted and stirring it with a spoon melts any last lumps. (If you take it too far, walk away and let it cool.)
Dip each radish into the butter, give it a shake, and dip it again. Lay it on a plate and repeat with the remaining radishes.
Sprinkle the radishes with the salt and saffron, and refrigerate for 30 minutes. The butter will harden and the seasonings will stay stuck to the surface of the radish.
Serve as party appetizers or an afternoon snack.