1. Heat the oven to 325 degrees. Grease a 9-inch round or square baking pan and line it with parchment paper so that the paper extends 2 inches beyond the pan. (This will create "handles" to remove the cake after baking.)
2. In a medium bowl, whisk together the potato starch, ground almonds, sugar and salt.
3. In a small bowl, whisk together the oil, eggs and vanilla. Pour the oil mixture over the dry ingredients and stir together with a wooden spoon until thoroughly combined and smooth. Stir in the melted chocolate and nuts. Pour the batter into the prepared baking pan and, if necessary, level with a frosting spatula dipped in cold water.
4. Bake 25 to 30 minutes until a toothpick inserted in the center comes out slightly moist but without crumbs. While still warm, cut into 8 equal pieces, leaving the cake in the pan. Set aside the cake to cool completely, then chill. Bring the cake back to room temperature before lifting it out of the pan (using the parchment handles), and gently separate into portions.