Prep the pan and oven: Heat the oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or grease well.
Mix dry ingredients: In a large bowl, whisk flour, protein powder, baking powder, baking soda, salt, and cinnamon.
Break up any lumps so the batter stays smooth.
Whisk wet ingredients: In a separate bowl, whisk eggs, Greek yogurt, milk, honey or maple syrup, oil, and vanilla until fully combined. Stir in lemon zest if using.
Combine gently: Pour wet ingredients into dry. Stir with a spatula just until no dry streaks remain. Do not overmix or the muffins will get tough.
Add berries: Fold in the berries gently.
If using frozen, add them straight from the freezer and toss with a teaspoon of flour first to reduce bleeding.
Fill the cups: Divide batter evenly among the muffin cups, filling nearly to the top for a tall rise. Sprinkle with coarse sugar or oats if you like.
Bake: Bake for 17–21 minutes, or until the tops are set and a toothpick comes out with a few moist crumbs (not wet batter). Larger berries may extend bake time by a minute or two.
Cool: Let muffins cool in the pan for 5 minutes, then move to a wire rack. Cool at least 15 minutes before eating so the crumb sets.