Preheat the oven to 200C or 400F. Grease your muffin pan with butter and oil or line the holes with muffin liners. This ensures the delicate muffins won’t get stuck to the pan.
In a small bowl, combine the milk and apple cider vinegar or lemon juice. Set it aside to curdle and form buttermilk.
While you’re waiting, combine all the dry ingredients in a large mixing bowl.
Now that the buttermilk is ready, add the yoghurt, melted butter and light brown sugar to the buttermilk bowl. Whisk until smooth.
Pour the wet ingredients into the dry ingredients and whisk to combine. The mixture should be about the thickness of a medium consistency yoghurt, and when left for a minute it should become a little bit light and bubbly. If it's thicker than this (keep in mind that flours vary and the optional psyllium/xanthan gum will make the batter thicker) add 60-80ml (1/4 - 1/3 cup) extra milk to reach a medium yoghurt batter consistency.
Add your add ins of choice to the mixture (reserving a few for topping) and use a spatula to gently fold them in. If you’re using frozen berries, gently squeeze them to remove excess liquid before adding to the bowl, and only ~just~ combine the batter. If you mix in the berry juice too much you might end up with grey muffins.
Spoon the batter into 10-12 muffin holes (this will depend on the yoghurt you use and the add ins) and top with the remaining add ins. As an option, you can finish the muffins with a sprinkle of finishing sugar.
Place the muffins in the oven for 20-25 minutes or until they are well browned on top and cooked through. Remove them from the oven and allow to cool for 5-10 minutes before very gently removing them from the muffin pan. Allow to cool completely on a baking rack before storing.
These muffins keep really well in an airtight container in the fridge for at least 3-4 days. They can also be frozen and defrosted before eating.