If you place the peppermint kisses on the cookies while they're still too hot, they will start to melt. So I recommend chilling the unwrapped kisses so that they're as cold as possible before placing them on the cookies. Also let the cookies cool for about 5 minutes before adding the kisses on top, and then immediately move them to a cooling rack. You can also let the cookies chill in the fridge after adding the peppermint kisses!