Preheat your oven to 350°F (180°C) and butter a mini bundt cakes pan (or a regular cake pan).
In a mixing bowl, combine sugar, butter, and egg. Mix well until the mixture turns pale yellow.
Add milk, mandarin orange zest, and salt. Mix until well combined.
Incorporate flour and baking powder into the mixture. Mix thoroughly.
Transfer the batter into the buttered pan and bake for 15 minutes (or until a toothpick comes out clean when pierced into a cake). About 30 minutes for a regular cake pan.
Allow the cakes to cool completely in the pan before removing them.
To Make the Glaze:
In a small bowl, mix confectioners' sugar and water.
Peel one mandarin orange and squeeze 2-3 peel pieces several times over the small bowl to release its oils (the outside of the peel should face the sugar mixture).
Drizzle the glaze over the cooled cakes and dust with confectioners' sugar before serving.