Preheat oven to 356º F (180° C). Line a muffin tray with cupcake or muffin liners. In a medium bowl, sift the flour and the cornstarch, baking powder and salt.
Separate the eggs and place the whites in a large clean, dry bowl and the yolks in another large bowl.
With an electric mixer or a hand whisk, beat the egg whites for 30 seconds until slightly fluffy, gradually add half of the sugar and continue beating for 2 more minutes until the whites turn foamy and soft peaks form.
Separately, with the same mixer, beat the egg yolks with the rest of the sugar, the melted butter, the juice and the mandarin zest for 2 minutes until the mixture is fluffy and slightly lighter in color.
Add in the flour mixture and beat for 30 seconds just until smooth.
Finally, add the beaten egg whites in three additions, mixing with a rubber spatula carefully in a folding motion until fully integrated into the mixture.
Pour the mixture into the muffin cups filling each one 3/4 of its capacity. Sprinkle a little sugar on top of each muffin if desired.
Bake the muffins at 356º F (180° C) for 20 minutes until lightly browned and cooked.
Remove the muffins from the oven and let them cool for at least 10 minutes. Decorate with powdered sugar or as desired.