Preheat oven to 180°C (350°F) and line a muffin tin with paper liners.
Whisk eggs and sugar – In a large bowl, beat eggs and sugar until light and fluffy.
Add wet ingredients – Mix in milk, oil, melted butter, yogurt, and vanilla sugar. Stir until smooth.
Incorporate dry ingredients – Sift in flour, baking powder, and salt. Gently fold until combined.
Add citrus zest – Stir in lemon or orange zest for extra flavor.
Fill muffin cups – Pour batter into muffin liners, filling each about 3/4 full.
Bake for 18–22 minutes or until a toothpick inserted in the center comes out clean.
Cool and serve – Let muffins rest for 10 minutes before transferring to a wire rack to cool completely.