In the top of a double boiler set over very hot water, melt the bittersweet chocolate and butter. Allow it to cool.
Using an electric mixer, beat the eggs and vanilla in a mixing bowl until frothy. Slowly add the sugar and beat until the mixture ribbons off the beaters. Add the melted chocolate mixture. Stir to combine.
Sift the dry ingredients together. Add to the chocolate mixture and stir to combine. Fold in the chips and nuts. The dough will seem more like cake batter than cookie dough. Cover the mixing bowl with plastic wrap and refrigerate until the dough is well chilled (and more cookie-dough-like).
Set the oven to 350 degrees. Line several baking sheets with baking parchment.
Scoop up the chilled dough and roll with your hands into small balls the size of a whole walnut (in the shell). Place the cookies on the prepared pans, about 1 1/2 inches apart.
Bake about 12 minutes or a bit longer. The tops of the cookies should look dry and cracked. Allow to cool for a minute on the baking sheet, then remove to cake racks for further cooling.
Recipe courtesy of the RIVER RUN COOKBOOK (HarperCollins, 2001)