Step 1

Preheat oven to 425ºF (220°C). Lightly grease a standard 12-cup muffin pan with butter or cooking oil spray.

Step 2

In a large bowl, whisk together flour, cocoa powder, white sugar, brown sugar, baking powder, baking soda, and salt. Stir in 3/4 of the chocolate chunks and coat in the dry mixture so they don't sink to the bottom while baking.

Step 3

Make a well in the middle and add in oil, eggs, sour cream, buttermilk and vanilla. Whisk the wet ingredients vigorously first, then use a wooden spoon to fold the wet mixture into the dry ingredients, until some of the bigger lumps have been smoothed out. The batter will be thick.

Step 4

Spoon the batter evenly into the prepared muffin cups just over 3/4 full (give the tops room to rise). Top with the remaining chocolate chunks.

Step 5

Bake muffins in the hot oven for about 8 minutes at 425°F (220°C), until muffins have puffed up slightly above muffin tray. Then reduce oven heat to 350ºF (175°C). Do NOT open oven when reducing temperature. Bake for a further 10-12 minutes or until a toothpick inserted in the centre of one comes out clean. The tops should be firm to the touch.

Step 6

Let muffins cool slightly in the pan (about 10 minutes) then transfer to a wire rack to cool completely.