Preheat the oven to 375 degrees F. Grease a standard muffin pan. This recipe makes enough batter for 16-18 muffins, so you can bake in batches and re-use the same pan, or grease an additional six cups in separate muffin pan, if you have one.
Finely grate enough mandarin oranges to get about 1/4 cup (30 grams) of zest. For me this was about 6 mandarins. You can also peel a long strip of zest with a vegetable peeler and julienne into small thin pieces for an optional garnish, as in my photos.
In a small saucepan over medium heat, combine the zest, 1/4 cup of the sugar, and the water. Heat and stir until the sugar has dissolved completely. Stir in the butter until melted. Remove from heat and set aside.
In a medium bowl, stir together the flour, remaining 1/4 cup of the sugar, baking powder, baking soda, and salt.
In a large mixing bowl, stir together the Greek yogurt, eggs, milk and orange sugar mixture until well combined.
Add the flour mixture to the yogurt mixture and stir until just combined.
Fill the prepared muffin pan(s) to 3/4 full. Bake for 15-20 minutes, until a toothpick inserted in the center comes back clean. Allow the muffins to cool in the pan for about 5 minutes, then remove from the pan to a cooling rack to cool completely.
Glaze (optional): In a small bowl, stir together the icing sugar, mandarin juice, zest and salt until smooth. If too thick for pouring, add a little juice until a pour-able consistency is achieved.
With a spoon, drizzle the cooled muffins with the glaze, and garnish with thin slices of mandarin peel if desired.