Make Orange Sugar: Mix sugar with zest, crushing zest into sugar with a spoon, until evenly mixed and sugar appears damp and light orange from the oils in the zest. Set aside; store in airtight container in the refrigerator indefinitely.
Heat oven to 400°F Grease bottoms of 12 standard muffin cups or line with baking cups.
Sift flour, baking powder, baking soda, and salt together into a bowl (or whisk together until well blended). Stir in 2/3 cup of Orange Sugar until evenly combined.
In a small bowl or measuring cup, whisk together milk and orange or lemon juice; milk will curdle and thicken slightly. In a medium bowl, lightly beat egg; whisk in 6 tablespoons melted and cooled butter, milk mixture, and vanilla.
Make well in center of dry ingredients and pour in wet ingredients. Stir just until blended. Divide evenly among prepared muffin cups.
Bake 20 to 25 minutes or until golden brown and toothpick inserted in center comes out clean. Cool on wire rack 10 minutes before removing from pan.
Place remaining 1/3 cup Orange Sugar in a custard cup or teacup; place 4 tablespoons melted butter in another custard cup or teacup. Dip tops of warm muffins first in melted butter, then in Orange Sugar, patting sugar on with a spoon if needed to form a sparkling sugar “crust” on top. This will get a little crunchy as the sugar dries. You'll probabably end with some butter and sugar left over.
Serve warm or store, loosely covered for a day or two (airtight storage will make the sugar squishy instead of crisp). Best either fresh and warm, or after sitting overnight.