Preheat the oven to 350°F (180°C). Line 2 or 3 large rimmed baking sheets with parchment paper.
In a medium saucepan over low heat, combine the dark chocolate and the butter and melt together, stirring regularly. Take off the heat and scrape into a bowl to cool for 5 to 10 minutes.
Put the dark muscovado sugar and caster sugar in a large clean bowl. Crack in the eggs. Beat with a handheld electric mixer until pale and fluffy, 3 to 4 minutes.
Slowly pour the melted chocolate and butter into the bowl and fold in gently. Sift over the flour, baking powder, baking soda, and salt, and mix until just combined. Stir in the milk chocolate chunks. This will be an unusually wet cookie mixture – definitely not something you can handle with your hands!
Working quickly, use a tablespoon or cookie scoop to take out a heaped scoop (2 to 3 tablespoons) of the mixture, and place on the prepared baking sheet.
Repeat with the rest of the mixture. You should be able to get 12 to 18 cookies altogether, 6 on each baking sheet, just make sure to leave plenty of room between the cookies, as they spread.
Sprinkle each cookie with a little granulated sugar, if desired, and bake until puffy and cracked, 12 to 14 minutes. Leave to cool for at least 15 minutes on the baking sheet – they will be very soft – and then transfer to a rack to finish cooling.