Equipment: One 10 by 15 by 1-inch pan buttered and lined with parchment or foil
Set a rack in the middle level of the oven and preheat to 375 degrees.
In a mixing bowl, combine the flour, spices and salt. Stir well to mix and sift the dry ingredients onto a piece of parchment or waxed paper.
In the bowl of an electric mixer, beat the butter with the sugar on low speed until just mixed. Beat in the chocolate, then the eggs, one at a time, beating smooth between each addition. Beat in the dry ingredients, scraping bowl and beater(s) several times. Off the mixer, fold in the chopped nuts. Scrape the batter onto the prepared pan and smooth the top with an offset metal icing spatula or a rubber spatula.
Bake the bars for about 30 minutes, until firm, but not dry. Cool in the pan on a rack. Trim 1/8-inch from the edges and cut the cooled sheet into 2- inch squares, using a sharp, thin-bladed knife.
Storage: Keep the bars between sheets of waxed paper in a tin or other container with a tight-fitting cover.