Preheat the oven to 180ºc and line 2 large or 3 small baking trays. Chop the dark chocolate into chunks and melt 50g of it. Put the melted chocolate to one side.
In a large bowl add the butter, light brown sugar and caster sugar. Using an electric whisk to cream the ingredients together.
Add the vanilla extract, milk and 50g of melted chocolate and whisk again until combined.
Now add the flour, baking powder and cacao powder and using a spatula combine well.
Save a handful of chopped walnuts and chocolate chunks and empty the rest into the mixture. Fold so they’re evenly scattered throughout.
Next split the mixture into approximately 16 balls and place them on the baking trays with plenty of space in between.
Flatten slightly and push the remaining chocolate and walnuts into the tops of the cookies.
Bake in the oven for 16-18 minutes and leave to cool slightly before sprinkling with sea salt flakes and enjoying!