Preheat oven to 200°C/390°F. Line a muffin/cupcake pan with liners.
Whisk sugar and butter till light and smooth.
Add eggs one at a time and mix in between till eggs are incorporated
Sieve in all the dry ingredients to the bowl and mix with a spatula until completely combined. Do not over-mix.
Add mix and vanilla and mix till properly combined.
Add chocolate chunks and mix till combined. The batter will be thick and sticky.
Spoon the batter into liners, filling them all the way to the top. Sprinkle some chocolate chunks on top.
Bake for 30-35 minutes at 180°C/350°F until a toothpick inserted in the center comes out clean.
Cool muffins for 10 minutes in the pan, then transfer them to a wire rack to cool completely.
Cover and store at room temperature for 5 days or in the refrigerator for 1 week.