Prep the oven and pan: Heat the oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease lightly.
Mix the dry ingredients: In a large bowl, whisk flour, protein powder, granulated sugar, brown sugar, baking powder, baking soda, and salt.
Whisk in the orange zest to release the oils.
Combine the wet ingredients: In a separate bowl, whisk orange juice, Greek yogurt, oil, eggs, and vanilla until smooth.
Bring it together: Pour the wet ingredients into the dry. Stir gently with a spatula just until a few streaks of flour remain. Do not overmix.
Fold in cranberries: Add cranberries and fold until evenly distributed.
The batter will be thick.
Fill the tin: Divide the batter among the muffin cups, filling each about 3/4 full. Sprinkle coarse sugar on top if you like a crunchy lid.
Bake: Bake for 16–20 minutes, or until the tops are set and a toothpick comes out with a few moist crumbs. If using frozen cranberries, add 1–2 minutes.
Cool: Let muffins cool in the pan for 5 minutes, then transfer to a rack to cool completely.
This helps set the texture.