Prep your pan and oven. Heat the oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or lightly grease it.
Mix dry ingredients. In a large bowl, whisk flour, protein powder, baking powder, baking soda, salt, and cinnamon until no lumps remain.
Whisk wet ingredients. In a separate bowl, whisk eggs, Greek yogurt, milk, oil, sugar, and vanilla until smooth and creamy.
The yogurt should be fully incorporated.
Combine gently. Pour the wet mixture into the dry ingredients. Stir with a spatula until just combined. Stop when you no longer see dry streaks.
The batter will be thick—avoid overmixing to keep the muffins tender.
Make the cinnamon swirl. In a small bowl, mix the swirl sugar, cinnamon, and melted butter until it resembles wet sand.
Layer the batter. Divide half the batter among the muffin cups. Add about 1/2 teaspoon of the cinnamon mixture to each, then top with the remaining batter. Dot the tops with any leftover swirl mixture and gently swirl with a toothpick.
Bake. Bake for 14–18 minutes, until the tops spring back when lightly pressed and a toothpick comes out with a few moist crumbs (not wet batter).
Protein bakes can go from perfect to dry fast, so start checking at 14 minutes.
Cool. Let muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely. This helps set the structure and prevents soggy bottoms.
Glaze (optional). Whisk the powdered sugar, milk, vanilla, and salt until smooth and pourable. Drizzle over cooled muffins.
For a thicker “frosting” vibe, use less milk.