Set out the egg and Greek yogurt to come to room temperature. This step is essential for fluffy muffins, so don’t skip it!
Melt the coconut oil and set it aside to cool back to room temperature. Be careful not to add it hot to the wet ingredients, as it can cause them to seize.
Preheat your oven to 350°F. Generously grease 9 cavities of a regular-sized, 12-cavity muffin pan with cooking spray or a little extra coconut oil. Dust the cavities with flour to prevent sticking. It’s crucial to skip paper liners for these muffins, as they tend to stick to them.
In a large bowl, combine the whole-wheat flour, cornstarch, baking powder, salt, and mini chocolate chips. Mix everything together until evenly distributed.
In another bowl, whisk together the vanilla extract, room-temperature egg, Greek yogurt, brown sugar, and the cooled coconut oil until smooth and creamy.
Gently fold the dry ingredients into the wet mixture, stirring until just combined. Be careful not to overmix, as that can lead to dense muffins. A few lumps are perfectly fine!
Divide the batter evenly among the 9 greased muffin cavities. If you want an extra touch of sweetness, sprinkle about 1/2 teaspoon of brown sugar on top of each muffin before baking. This will give them a nice crunch!
Fill the remaining cavities halfway with water to ensure even baking. This is a great trick to prevent the muffins from drying out.
Bake in the preheated oven for 18 to 22 minutes, or until the edges are light brown and a toothpick comes out clean or with a few moist crumbs. Be cautious not to overbake, or the muffins will lose their softness.
Once baked, remove the muffins from the oven and allow them to cool in the pan for about 5 minutes. Then, carefully transfer them to a cooling rack to cool completely, ensuring air can circulate around all sides.