Preheat and Prepare: Preheat your oven to 180°C (350°F).

Line a muffin tin with paper liners or lightly grease with butter or cooking spray.

Mix Wet Ingredients: In a large mixing bowl, whisk together the eggs and white sugar until light and creamy.

Add the cold milk, sunflower oil, and melted butter. Mix well.

Stir in the yogurt and vanilla sugar, ensuring the mixture is smooth and well combined.

Combine Dry Ingredients: In a separate bowl, sift together the flour, baking powder, and salt.

Stir in the lemon or orange zest to distribute it evenly.

Combine Wet and Dry Ingredients: Gradually fold the dry ingredients into the wet mixture using a spatula or wooden spoon. Mix gently until just combined; avoid overmixing to maintain a fluffy texture.

Fill and Bake: Spoon the batter into the prepared muffin tin, filling each cup about 2/3 full to allow room for rising.

Bake in the preheated oven for 18-22 minutes, or until a toothpick inserted into the center comes out clean.

The tops should be golden and spring back when lightly pressed.

Cool and Serve: Allow muffins to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.

Enjoy warm or at room temperature, plain or with butter, jam, or honey.