Preheat oven to 375°F (190°C) and line a muffin tin with paper liners or grease well. In a large bowl, whisk together the flour, baking powder, baking soda, and salt.
In a separate bowl, whisk the sugar, egg, sourdough discard, oil or melted butter, orange juice, vanilla, and orange zest until smooth.
Add the wet ingredients to the dry ingredients and gently fold just until combined. Do not overmix.
Divide the batter evenly between muffin cups, filling about ¾ full. Bake for 18 to 22 minutes, or until lightly golden and a toothpick inserted into the center comes out clean. Cool muffins in the pan for 5 minutes, then transfer to a wire rack to cool slightly.
In a small bowl, whisk powdered sugar with 15 g orange juice, adding more as needed until a pourable glaze forms. Stir in optional orange zest. Spoon glaze over warm or fully cooled muffins.
Spoon glaze over warm or fully cooled muffins.