Prep the oven and pan: Heat oven to 375°F (190°C). Line a 12-cup muffin tin with liners or lightly grease.
Mix dry ingredients: In a large bowl, whisk flour, protein powder, baking powder, baking soda, salt, and cinnamon until well combined.
Whisk wet ingredients: In a separate bowl, whisk eggs, Greek yogurt, maple syrup, oil, milk, and vanilla until smooth.
Combine: Pour wet ingredients into dry.
Stir gently with a spatula until just combined. The batter should be thick but scoopable. If it’s too stiff, add 1–2 tablespoons more milk.
Fold in berries: Toss blueberries with 1 tablespoon flour, then fold into the batter with a few gentle strokes.
Don’t overmix.
Fill the tin: Divide batter evenly among the cups, filling to the top for tall domes. Sprinkle with coarse sugar or sliced almonds if using.
Bake: Bake 18–22 minutes, until the tops are set and a toothpick comes out clean or with a few moist crumbs.
Cool: Let muffins cool in the pan 5 minutes, then transfer to a rack to cool completely. This helps the texture set and prevents soggy bottoms.