First, make the homemade buttermilk by stirring 1 teaspoon of vinegar into 180 ml of warm milk. Let it stand for 5–10 minutes until it thickens and slightly curdles. Meanwhile, preheat your oven to 220°C (425°F) and line a 10- or 12-cup muffin tin with paper liners.
In a large mixing bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt until everything is evenly combined. This helps distribute the leavening agents evenly for tall, fluffy muffins.
In a separate bowl, whisk together the melted butter and sugar until smooth. Then add the eggs one at a time, followed by the prepared buttermilk, Greek yogurt, and vanilla extract. Continue whisking until the mixture is smooth and fully combined.
Gradually pour the wet ingredients into the dry ingredients. Using a spatula, gently fold everything together just until no dry streaks remain. Avoid overmixing, as this can make the muffins dense instead of light and tender.
Next, gently fold the chocolate chips into the batter, making sure they are evenly distributed without overworking the mixture.
Divide the batter evenly between the prepared muffin liners, filling each about ¾ full. Finally, sprinkle a generous handful of extra chocolate chips over the tops for a bakery-style finish.
Bake at 220°C (425°F) for 5 minutes. Then, without opening the oven door, reduce the temperature to 180°C (350°F) and continue baking for 15–18 minutes, or until the muffin tops are golden and a toothpick inserted into the center comes out with a few moist crumbs.
Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely. Enjoy them warm while the chocolate is still melty or store them for later.