Preheat your oven to 190°C (375°F). Line a 12-cup muffin tin with paper muffin cases or lightly grease each cup with butter or cooking spray. If you’re using frozen blueberries, keep them in the freezer until you’re ready to add them to the batter. Preparing everything before mixing makes the baking process easier and helps prevent the batter from sitting for too long.

In a large mixing bowl, add the all-purpose flour, granulated sugar, baking powder, baking soda, and salt. Whisk the ingredients together until evenly combined, breaking up any small flour or sugar lumps. Mixing the dry ingredients separately helps distribute the raising agents throughout the batter, allowing the muffins to rise more evenly during baking.

In another bowl, whisk together the melted butter, eggs, sour cream, milk, and vanilla extract until smooth. Make sure the melted butter has cooled slightly before adding the eggs so it doesn’t scramble them. Stir in the optional lemon zest for a fresh citrus flavour. The mixture should look creamy and smooth, with all the ingredients properly incorporated.

Pour the wet mixture into the bowl of dry ingredients. Use a silicone spatula to gently fold everything together until just combined. A few small lumps are perfectly fine, so avoid mixing until the batter becomes completely smooth. Gently fold in the blueberries, taking care not to crush them. If using frozen blueberries, add them straight from the freezer to help prevent the batter from turning purple.

Divide the batter evenly between the 12 prepared muffin cases, filling each approximately three-quarters full. Use an ice cream scoop or large spoon to make the portions consistent. Add a few extra blueberries on top of each muffin and sprinkle with a little granulated sugar for a lightly crisp finish. Avoid overfilling the cases because the muffins need room to rise.

Place the muffin tin in the preheated oven and bake for 20–25 minutes, or until the tops are golden and a toothpick inserted into the centre comes out clean or with a few moist crumbs. Remove the tin from the oven and allow the muffins to cool for approximately 5 minutes. Transfer them to a wire rack to cool further before serving.