Preheat oven to 190C/375F. Grease 12 muffin cups or line them with liner papers.
In a large bowl, toss together sugar and orange zest. Add flour, baking powder, and salt, and mix to combine. In another medium bowl, whisk eggs with melted butter, sour cream, milk, orange juice, and vanilla extract until well combined. Make a well in the dry ingredients and pour the wet ingredients into it, then stir with a wooden spoon or rubber spatula just until combined. Do not over mix. Fold in add-ins.
Divide batter evenly between muffin cups (almost all the way to the top). Bake for 15-20 minutes or until a toothpick inserted into the center of the muffin comes out clean. Allow to cool for 10 minutes, then remove muffins from the tin and transfer to a wire rack to cool completely.
To make the orange drizzle topping: In a small bowl, sift powdered sugar. Add orange juice, a bit at a time, and whisk until you get a thick yet pourable consistency. Once muffins have cooled, pour glaze over the tops using a tablespoon, letting it drizzle down the sides.
Muffins are best the same day they are made, but can be stored in an airtight container at room temperature for an additional day. Muffins can be frozen for up to 2 months.