Preheat your oven to 170C / 340F (fan off), 150C / 300F (fan on). Line a cupcake tin with 12 brown cupcake liners or muffin liners. Using muffin liners will result in larger muffins and you’ll only get 8 muffins.
Combine dry ingredients - To a large mixing bowl, add the flour, cocoa powder, bicarb soda, salt and sugar and use a whisk to combine.
Coat chips in dry ingredients - To a medium sized mixing bowl, add half the milk chocolate chips and dark chocolate chunks and a tablespoon of the dry ingredients and mix to coat. This will prevent the chocolate chunks sinking to the bottom of your muffins as they bake.
Form batter - To the bowl with the dry ingredients, add the melted butter, Greek yogurt, eggs, buttermilk and whisk to combine. Then add the boiling water and whisk until well combined.
Bake - Add the chocolate chunks and use a spatula to fold through. Distribute the batter amongst 12 cupcake liners or 8 muffin liners. Bake cupcake liner muffins for 16 minutes and muffin liner muffins for 22 minutes. Once baked, take out of the oven, add remaining chocolate chunks on top of the muffins leaving the center clear for the ganache. Transfer to a cooling rack to cool down completely.
Add ganache (optional) - Once cooled, pipe or spoon the ganache on top of the muffins. Serve.