Preheat oven to 180°C | 360°F.Spray 2 muffin trays with spray oil and wipe over with paper towel.In a large sized bowl, mix together all wet ingredients.Add dry ingredients, mixing again, but be careful not to over beat the batter.Fold in blueberries and chocolate chips.Pour ¼ cup batter per muffin into each muffin cup and top with extra chocolate chips.Bake in pre-heated oven for approximately 25 - 30 mins, or until muffins are golden brown on top and cooked through when tested with a toothpick