Preheat the oven to 350 degrees F. Oil a 6-inch by 3-inch round cake pan. In a large bowl, combine the flour, sugar, cocoa powder, baking soda, salt, fennel, and cardamom and whisk to combine. In another bowl, whisk together the water, vegetable oil, and vinegar and then add to the dry ingredients. Mix well and pour the batter into the prepared cake pan. Bake for 1 hour. The cake is done when a toothpick comes out clean when inserted into the center of the cake. Cool on a rack and serve.