Prep the oven and pan. Preheat to 350°F (175°C). Line a 12-cup muffin tin with liners or lightly grease.
Combine dry ingredients. In a large bowl, whisk flour, protein powder, baking powder, baking soda, salt, and cinnamon.
Break up any clumps, especially from the protein powder.
Whisk the wet ingredients. In a separate bowl or large measuring cup, whisk eggs, milk, yogurt, oil, sugar, and vanilla until smooth.
Bring it together. Pour the wet mixture into the dry ingredients. Stir with a spatula until just combined. The batter should be thick but scoopable. Do not overmix.
Fold in extras. If using berries or chocolate chips, gently fold them in now.
Fill the tin. Divide the batter evenly among the muffin cups, about 3/4 full.
Bake. Bake for 14–18 minutes, or until the tops spring back and a toothpick comes out with a few moist crumbs.
Cool. Let the muffins rest in the pan for 5 minutes, then transfer to a rack to cool.
This helps set the texture and prevents soggy bottoms.