Step 1Heat the oven to 350 degrees. Generously grease an 8x4 1/2-inch loaf pan.
Step 2Whisk together the oil, granulated sugar, brown sugar, Passover vanilla sugar and cocoa in a mixing bowl. Whisk in the eggs, then stir in the coffee, salt, matzo meal, matzo cake meal and potato starch. Let stand 10 minutes. Fold in the chocolate and the nuts. Spoon the batter into the pan.
Step 3Bake the loaf until the top is set, about 35 to 40 minutes. Wrap the loaf well in foil and refrigerate for a couple of hours to firm up.
Step 4Heat the oven to 325 degrees. Cut the loaf into slices, about 1/4 inch thick. Place them on a parchment paper-lined baking sheet and then bake them again to dry them out, turning the slices halfway through, about 15 to 20 minutes.