Pre-heat the oven to 180°C / 160°C fan assisted / gas mark 4 / 350°.
Roughly chop the dark chocolate into small chunks and set aside.
In a large mixing bowl (or stand mixer), combine the caster sugar, soft light brown sugar, eggs, melted and cooled unsalted butter, milk, and vanilla extract.
Use an electric hand mixer to thoroughly mix these wet ingredients (or set your mixer to low) for about a minute until the mixture is smooth and well combined.
Add the dry ingredients (flour, baking powder and salt) to the bowl with the wet ingredients.
Mix well until the batter is thick and smooth.
Gently stir in 180g of the chocolate chunks into the muffin batter until they are evenly distributed.
Line a 12- hole regular muffin tin with paper cases.
Spoon the muffin batter into each case, filling them about two-thirds full. There is enough batter for 16 muffins. You can either start a second muffin tin (if you have one) or you can rest the remaining batter until the first batch of muffins has been baked and you can bake the final 4 muffins afterwards.
Scatter the remaining chocolate chunks over the top of the muffins.
Place the muffin tin in the pre-heated oven and bake the muffins for 25 minutes, or until they are golden brown and a toothpick inserted into the centre of a muffin comes out clean.
Once baked, remove the muffins from the oven and transfer the muffins immediately to a wire rack to cool completely.