Preheat the oven to 180°C (350°F) and make eight muffin cases by cutting 12 cm (5 inches) squares of baking paper and pressing them over a cup whose base fits into your muffin tray holders. Place the muffin cups into the muffin tin.
Into a large mixing bowl, sift together the flour and salt.
Into a separate bowl, add the eggs and sugar and beat lightly with a hand whisk until combined.
Add the sour cream, melted butter, oil, and vanilla to the egg mixture. Whisk together until well mixed.
Make a well in the center of the dry ingredients and pour in the wet ingredients. Use a wooden spoon to mix, turn the mixture only 5 times.
Add the chocolate and blueberries to the muffin batter, and mix with a wooden spoon until combined. Do not over-mix, in total the batter should only be turned about 12 times.
Use an ice cream scoop to fill the muffin cases two-thirds of the way up the paper. Sprinkle the tops of the muffins with extra sugar.
Bake in a preheated oven for about 25 minutes until the muffin tops are golden and a skewer inserted into the center of the muffins comes out clean. Allow the muffins to cool on a wire rack for about 10 minutes, serve warm and enjoy!