Madeleine cookies are best the day they are made; however, they will keep for 2-3 days at room temperature or refrigerated in an airtight container. If you choose to refrigerate them (giving the madeleines a longer shelf life), you should bring them to room temperature before serving, and wait to sprinkle with powdered sugar until ready to serve. Also, note that madeleines are quite delicious dunked in coffee or tea—highly recommended if your madeleines are a little bit stale!