Preheat the oven to 350°F with a rack in the upper and lower thirds.
Line two cookie sheets with parchment paper.
Cut ¼ of the bars (12 squares) into ¼-inch chunks, set the chunks aside separately.
Break remaining ½ bar of chocolate roughly, put pieces in medium heatproof bowl and set directly in a wide skillet of almost simmering water. Add butter.
Stir the chocolate and butter until smooth and fully melted.
Remove bowl from skillet and stir in both sugars, salt, baking soda, and vanilla.
Add the egg and stir briskly until well blended.
Add all of the flour and stir just until it is entirely blended - the dough will be stiff.
Let dough cool for a few minutes before stirring in the nuts and reserved chocolate chunks.
Scoop about 2 Tbsp. of dough for each cookie and place them 2 inches apart on lined cookie sheets.
Bake 10-14 minutes, until cookies are puffed and cracked. Rotate the sheets from upper to lower half way through the baking time. Set pans on racks to cool.