Wash the mandarin oranges throughly. Slice mandarins into ¼" rounds (I eyeballed it).
In a saucepan, combine water and sugar. Heat on high until sugar is dissolved and mixture is boiling. Add mandarin slices and reduce heat to medium-low. Make sure the mandarins are completely submerged, then cover and let simmer for 10 minutes.
Remove from heat and let slices cool for 10 minutes. Using a slotted spoon, transfer the slices to a cooling rack. Let cool and then they are ready to use on the cookies, or to eat on their own!
Recipe by the modern barista at https://themodernbarista.com/index.php/2015/11/16/christmas-reserve-with-mulled-spice-cookies/