Preheat your oven to 425 °F (220 °C) and line your chosen muffin tray with with muffin liners or baking paper squares. Spray with cooking spray and set aside.
In a large mixing bowl, whisk together the dry ingredients and sugar until no clumps remain.
2 cups all-purpose flour, ¾ cup cocoa powder, 2 ½ tsp. baking powder, ½ tsp. baking soda, ½ tsp. sea salt, 1 ¼ cups granulated sugar
In a large measuring jug or glass bowl, whisk together the wet ingredients.
1 ¾ cups buttermilk, ⅓ cup melted butter, 2 Tbsp. vegetable oil, 1 tsp. vanilla extract
Pour the wet ingredients into the dry ingredients and fold together until just combined and no dry spots remain. Be careful not to over-mix, the muffin batter should have some lumps in it.
Fold in most of the chocolate chips and chunks until just combined into the batter.
6 oz. chocolate chips and chunks
Divide the batter equally between the greased muffin cups. Fill them about 80% - 90% full and sprinkle the reserved chocolate chips and chunks on top.
Bake the muffins for 3 minutes, then reduce the oven temperature immediately to 350 °F (180 °C) and bake for a further 15 - 17 minutes until a cake tester or toothpick comes out clean and crumb-free.
Remove the muffins from the oven and allow to cool in the muffin tray for only 5 minutes. They should then be easier to remove and place on a wire rack to cool further.
Serve muffins once cooled or store for the next few days!