In a mixing bowl, add Self-Rising Flour (1 1/2 cup), Caster Sugar (1 cup), Unsweetened Cocoa Powder (1/2 cup), and Semi-Sweet Dark Chocolate (100 gram). Mix until combined.
In a mixing jug, add Buttermilk (1 cup), Butter (2.8 fluid ounce), and Egg (1). Whisk to combine.
Make a well in the center of the dry ingredients and pour in wet ingredients. Gently mix together until just combined.
Divide the batter between the holes of a 12-hole muffin tin lined with paper cases. Sprinkle Semi-Sweet Dark Chocolate (100 gram) over the top of each muffin and press down lightly.
Bake at 400 degrees F (200 degrees C) for 20 minutes.
Allow muffins to cool in pan for 10 minutes.