Coarsely chop 2 ounces of the chocolate and then melt it in the top of a double boiler over not-quite-simmering water. Remove the pan from the heat. Whisk the chocolate, to smooth. Set aside. Using an electric mixer, cream the butter, peanut butter, and brown sugar. Add the egg, beating until smooth, then blend in the vanilla. Sift the flour, baking soda, and salt into a medium bowl. Set aside. Blend the melted chocolate into the creamed mixture until evenly combined. Stir in the dry ingredients, a third at a time, until evenly mixed. Coarsely chop the remaining 4 ounces of chocolate and stir it into the dough along with the peanuts. Cover and refrigerate the dough for 15 to 20 minutes.
Preheat the oven to 350°F. Lightly butter two large baking sheets or line them with parchment paper. Using floured hands, roll the dough into 1½-inch-diameter balls and place on the baking sheets, leaving about 2½ inches in between. Bake one sheet at a time on the center oven rack for about 15 minutes. When done, the tops of the cookies will be cracked, and they’ll still feel soft when pressed gently. Do not overbake. Cool the cookies for 2 to 3 minutes on the baking sheet, then transfer to a rack to cool completely.