Preheat oven to 180 C or 375 F. Grease a standard 12 hole muffin tin with cooking spray, or line with paper cases.
Sift flour, caster sugar, cocoa powder, baking powder, baking soda, and salt into a large bowl. Set aside.
Combine butter, eggs and milk together in a large bowl. Set aside.
Make a well in the center of the flour mixture. Pour the milk mixture in it. Using a large metal spoon, mix gently until just combined. Fold in 1 tablespoon chocolate chips.
Spoon the batter evenly into the holes of the prepared muffin tin. Top each with more chocolate chips.
Bake chocolate muffins in preheated oven for 15 – 20 minutes, or until a toothpick inserted into the center of the muffin comes out clean. Cool muffins in the pan for 10 minutes before transferring to a wire rack to cool completely.
Store the muffins in an airtight container.