Preheat your oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease it well.
In a large bowl, whisk together the flour, sugar, baking powder, and salt. This is your dry mix – set it aside for now.
In another bowl, combine the melted butter, eggs, milk, orange juice, and orange zest. Whisk until well blended. This is your wet mix.
Now, here's where the magic happens! Pour the wet ingredients into the dry ingredients. Using a spatula or wooden spoon, gently fold the mixtures together. Stop when you see just a few streaks of flour remaining – we don't want to overmix!
Fold in the fresh cranberries (and any optional add-ins) until just combined. The batter should look lumpy – that's perfect!
Divide the batter evenly among the muffin cups. They should be about 2/3 full.
Bake for 18-22 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Let the muffins cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.