Melt the chopped chocolate in the top of a double boiler over not-quite-simmering water. Whisk briefly to smooth, then remove the pan from the heat. Set aside. Using an electric mixer, cream the butter in a large bowl, gradually adding the sugars. Beat in the eggs, one at a time, then blend in the vanilla. Slowly blend in the melted chocolate. Sift the flour, cocoa, baking soda, and salt into a separate bowl. Stir the dry ingredients into the creamed mixture, about half at a time. Stir in the nuts and chocolate chips, until evenly mixed. Cover the dough and refrigerate for 30 minutes.
Meanwhile, preheat the oven to 350°F. Butter two large baking sheets or line them with parchment paper. With lightly floured hands, roll the dough into 1½- to 1¾-inch-diameter balls. Place on the baking sheets, leaving 2 inches in between. Bake one sheet at a time on the center oven rack for 13 to 14 minutes. When done, the cookies will still seem squishy-soft when pressed gently. Cool on the baking sheet for 5 minutes, then transfer to a rack and cool completely.