Preheat the oven to 350°F (175°C) and prepare a muffin tin with liners or nonstick spray.
In a large bowl, whisk together flour, sugar, baking powder, and salt.
In another bowl, beat the egg and mix in the milk, melted butter, orange zest, and orange juice.
Combine wet ingredients with dry ingredients; gently fold until just mixed.
Fill muffin tins two-thirds full with batter.
Bake for 18-20 minutes until golden brown; check doneness with a toothpick.
For the glaze, mix powdered sugar with orange juice until smooth. Drizzle over warm muffins before serving.