Preheat the oven to 390°F (200°C) and spray a muffin tray with non-stick spray. Set aside.
Sift the all purpose flour into a mixing bowl and add the white and brown sugar, baking powder, baking soda and salt. Mix together.
Add buttermilk to a measuring jug and then add the eggs, melted butter, canola oil and vanilla extract and whisk together until mixed.
Pour this out over the dry ingredients and mix into a thick batter. Don't overmix.
Add 1 cup of chocolate chips and fold them into the batter.
Divide the batter evenly between the muffin partitions of your prepared muffin tray.
Add the remaining ¼ cup of chocolate chips directly to the tops of each muffin.
Place them into the oven and bake at 390°F for 5 minutes. Then turn the temperature down to 350°F and bake for a further 15 minutes until lightly browned on top and a toothpick inserted into one of the muffins comes out clean (melted chocolate is okay though).
Allow the muffins to cool for a few minutes and then transfer them to a wire cooling rack to cool completely.