Preheat your oven to 220°C/200°C fan.
Place your muffin cases/liners in your 12-hole muffin pan, or if not using cases, spray the muffin holes with non-stick cake pan spray. Set aside.
To your large mixing bowl add the plain flour, baking powder, bicarbonate of soda, salt, caster sugar and light brown sugar. Whisk until well combined and set aside.
To your medium mixing bowl add the melted butter, large eggs, vanilla extract, sunflower oil and buttermilk. Whisk until well combined.
Pour this wet mixture into the large bowl of dry ingredients and using a spatula or wooden spoon, fold together until no pockets of flour remain. Be careful not to overmix at this stage to avoid dense, heavy muffins.
Add the chocolate chips and gently fold into the muffin batter. The batter will be quite thick.
Fill each muffin case/hole 3/4 full with muffin batter.
Place into your preheated oven and set a timer for 5 minutes.
After 5 minutes, reduce the oven temperature to 180°C/160°C fan and continue baking for 12-15 more minutes.
To check if the muffins are done, insert a cake skewer or cocktail stick into the centre of the muffins and check if it comes out clean. If it contains any wet mixture, place the muffins back in for a few more minutes before checking again.
Once baked remove from the oven and place the tray onto a wire cooling rack. After 5 minutes, remove the muffins and place them onto the wire cooling rack to finish cooling completely.
Best eaten within around 4 days. Store in an airtight container at room temperature.