Melt the unsweetened chocolate, 4 ounces of the bittersweet or semisweet chocolate, and the butter in a small saucepan over low heat (or in a bowl in the microwave) and set aside to cool slightly.

Using a stand mixer with the whisk attachment on medium-high speed, whip the eggs and sugar until thick and light, about 10 minutes. With the mixer on low speed, add the melted chocolate mixture and the vanilla and mix until blended.

With a rubber spatula, fold in the flour, baking powder, and salt and scrape the bowl. Fold in the remaining chopped chocolate and the walnuts. Refrigerate the dough until it has firmed up enough to scoop, about 1-1/2 hours.

Position a rack in the center of the oven and heat the oven to 350 degrees F. Drop the chilled dough by rounded tablespoonfuls about 3 inches apart on ungreased cookie sheets. Bake until the cookies are cracked on top and feel dry on the surface but still soft inside, about 15 minutes. Let the cookies cool on the sheet for 1 minute before transferring them to a rack to cool.

The cookies can be stored in an airtight container for up to three days or frozen for about one month.