Move the oven rack to the middle position. Preheat the oven to 425°F (220°C). Grease two muffin pans with nonstick spray or line them with muffin papers. Set aside.
In a large bowl, whisk together the 2 ½ cups all-purpose flour, 2 tablespoons cornstarch, 1 tablespoon baking powder, ½ teaspoon baking soda, and ½ teaspoon kosher salt until well combined.
In a separate bowl, add the 1 ½ cups granulated sugar and the zest from 2 large oranges. Massage and pinch to zest into the sugar.
Slice off the top and bottom of the oranges. Stand the oranges up with the cut side down. Starting at the top of an orange, slice the peel away following the curve of the orange. Continue slicing all the way around the orange. Slice alongside the internal membranes to remove each segment of orange. Repeat the supreming process with the second orange.
Slice the orange segments into smaller pieces and add them to the flour mixture. Whisk to coat them with flour.
Add the 1 cup sourdough discard, ⅔ cup neutral oil, ½ cup buttermilk, 3 large eggs, and 1 tablespoon vanillato the bowl containing the sugar and orange zest. Mix until smooth.
Pour the wet ingredients into the dry ingredients and and add the white chocolate chips. Stir until just combined.
Divide the muffin batter evenly among the muffin cups filling nearly to the top.
Bake the muffins in the preheated oven for 5 minutes, then, keeping the muffins in the oven, reduce the oven temperature to 350°F (180°C) and continue to bake for 13 to 16 minutes until the tops are golden brown.
Allow the muffins to cool for several minutes in the muffin pan before transferring them to a wire rack to cool completely.
Orange Icing
Whisk together the 1 ½ cups powdered sugar, ⅛ cup orange juice, and ⅛ teaspoon kosher salt until smooth.
Drizzle the icing over the muffins. You can either use a spoon to drizzle the icing or spoon it into a plastic zip-top and cut the corner off the bag to drizzle the icing across the muffins.
Notes
Storage: These muffins will stay moist for a couple of days when stored at room temperature. Leftover muffins can be stored in an airtight container for 2-3 days.For longer storage keep them in the refrigerator for up to a week.They can also be frozen for up to 3 months. Wrap each muffin in plastic wrap and place them in a zip-top bag. Thaw muffins at room temperature.Tips
Muffins bake best on the middle rack of the oven. Too low and the bottoms will burn, too high and the tops will brown too quickly.
Make sure the orange pieces are coated with the flour mixture before adding the wet ingredients. This will ensure they are evenly distributed throughout the batter and they won't sink to the bottom.
When zesting an orange, you only want to use the outer orange layer, not the bitter white pith.
When supreming an orange, use a sharp knife, slice slowly, and never force the knife. Your fingers and knife are working closely together and this is the best way to avoid an accident.
Don’t over-mix the batter. A few lumps are fine. Overmixing will create too much gluten causing your muffins to be tough.
Use a cookie scoop to evenly portion the cupcake batter into the muffin pans.
Use a toothpick to test for doneness. Insert a toothpick into the center of a muffin, if it comes out clean, the muffins are done.
Add additional orange zest to the icing for a stronger orange flavor.