Preheat oven to 325 degrees and set rack in the middle level of the oven. Cover 2 large cookie sheets or jelly roll pans with parchment or foil.
Mix the flour, cocoa, baking powder and salt and sift into a mixing bowl. Stir in the sugar and nuts.
Whisk the eggs and vanilla and stir into the flour mixture to form a dough.
On a lightly floured surface, press dough together. Divide dough in half and roll into a log the length of the pan (14 to 18 inches) Place each log on the pan and flatten slightly. Bake for about 30 minutes, until well risen and firm. Cool the logs on the pans.
After the logs have cooled, detach from paper and slice them about 1/2-inch thick with a sharp serrated knife. Place back on paper lined pans, cut side down, and bake again for about 20 minutes, or until dry and crisp. Cool on pans and store in a tin or plastic container with a tight fitting lid.