Add 400 grams of dark chocolate to a bain marie. Heat, stirring frequently, until completely melted. Remove from the heat.
In a bowl, stir together flour, baking powder and salt and set aside. Crack the eggs into another bowl and set aside. Use a mixer to beat the butter on medium speed until very light and fluffy. Add the sugar and beat until well blended. Add the eggs and beat briefly to mix. Then add the melted chocolate and combine.
On a low speed, add the flour mixture a little at a time, beating after each addition until incorporated before adding more. Remove the bowl from the mixer and hand fold the finely chopped chocolate and walnuts into the mixture with a rubber spatula.
To shape the cookies, scoop out heaped tablespoons of dough, form into balls, and place on prepared baking sheets, spacing the balls about 1 inch apart. Bake for about 15 minutes or until set around the edges. Remove from oven and transfer the cookies to a wire rack to cool slightly. Serve warm.
Leftover cookies can be stored, airtight, at room temperature for 3 days, or, if tightly wrapped, frozen for up to 1 month.