Preheat the oven to 425ºF (220ºC). Lightly grease a standard 12-cup muffin tin with vegetable oil, line the cups with paper muffin liners, and set aside.
In a large mixing bowl, whisk together the flour, sugar, baking powder, orange zest, and salt. Gently fold in the blueberries and chocolate chips until they are well coated in flour and evenly distributed.
In a separate mixing bowl or large measuring cup, whisk together the milk, vegetable oil, vanilla, and egg until smooth.
Use a sturdy spoon or spatula to make a well in the center of the dry ingredients. Pour the wet ingredients into the well, then gently fold until just combined. A few streaks of flour remaining in the batter is fine
Divide the batter evenly between the 12 prepared muffin cups. The cups will be quite full, but this is the correct amount. Sprinkle a few extra blueberries and chocolate chips on top of each muffin, if desired.
Place the muffin tin in the oven and bake for 10 minutes. Without opening the oven door, reduce the heat to 350ºF (180ºC) and continue baking for another 8 to 10 minutes, 18 to 20 minutes total, or until the tops are golden and spring back when lightly pressed with a fingertip.
Notes
Use a digital kitchen scale for accurate measurements and an oven thermometer to be sure your oven is heating properly.
Zest the orange directly over the bowl of dry ingredients so you catch every bit of the oils.
Stop stirring as soon as the flour is mixed in, even if the batter looks a little lumpy.
A cookie or ice cream scoop is terrific for portioning the batter into the muffin cups.
Storage
Store cooled muffins in an airtight container at room temperature for up to two days or in the fridge for up to five days. They can also be frozen for up to two months and thawed at room temperature.