Preheat oven to 350°F (175°C). Line a muffin tin with 12 paper liners.
Mix dry ingredients: In a bowl, whisk together flour, sugar, baking powder, baking soda, salt, and cinnamon.
Prepare wet ingredients: In another bowl, whisk eggs, orange juice, melted coconut oil, orange zest, and vanilla until smooth.
Combine: Gently fold the wet mixture into the dry ingredients until just combined. Don’t overmix.
Add cranberries: Fold in cranberries (and walnuts if using). Batter will be thick.
Fill muffin tins: Divide batter evenly among 12 liners, filling each about ¾ full.
Bake for 18–22 minutes, until a toothpick in the center comes out clean.
Cool and enjoy: Let muffins cool in the pan for 10 minutes, then transfer to a rack.