Preheat oven to 375°F (190°C). Line a muffin pan with liners.
Mix dry ingredients in a large bowl: flour, sugar, baking powder, baking soda, and salt.
Whisk wet ingredients in a separate bowl: melted butter, eggs, milk, and vanilla bean seeds.
Combine gently by pouring wet ingredients into dry. Stir just until no dry pockets remain.
Fold in papaya carefully so it doesn’t break down too much.
Divide batter evenly into muffin cups, filling about ¾ full.
Top if desired with coarse sugar.
Bake for 18–22 minutes, until tops are lightly golden and a toothpick comes out clean.
Cool 5 minutes in the pan, then transfer to a rack.