Preheat and prep: Heat the oven to 350°F (175°C).
Line a 12-cup muffin tin with paper liners or grease well.
Whisk dry ingredients: In a large bowl, whisk the gluten-free flour, protein powder, baking powder, baking soda, salt, cinnamon, and optional flax/chia until well combined. Break up any protein powder clumps.
Mix wet ingredients: In another bowl, whisk Greek yogurt, eggs, milk, oil, maple syrup, and vanilla until smooth.
Combine gently: Pour the wet mixture into the dry. Stir with a spatula just until no dry streaks remain.
The batter should be thick but scoopable. If too thick, add 1–2 tablespoons milk.
Fold in mix-ins: Gently fold blueberries, chocolate chips, or nuts into the batter. Avoid overmixing to keep the crumb tender.
Portion: Divide batter evenly among the 12 cups, filling each about 3/4 full.
For taller domes, you can fill slightly higher.
Bake: Bake 16–20 minutes, or until the tops are set and a toothpick comes out mostly clean with a few moist crumbs.
Cool: Let muffins cool in the pan for 5 minutes, then transfer to a wire rack. Cooling helps them set and prevents a gummy texture.
Enjoy: Serve warm or at room temperature. Add a smear of nut butter for extra protein and healthy fats.