In boiling water, cook the beets whole and unpeeled. Cook until tender and easily pierced with a sharp knife, 30-40 minutes. Drain, then cool under running water. When cool enough to handle, trim stem and root ends and peel. In a food processor or blender, process into a smooth purée.

Butter a 9-inch round cake pan and line with a round of baking parchment. Preheat oven to 350˚F.

In a large bowl, whisk together the flour, sugar, cocoa powder, baking soda, and salt.  Whisk in eggs, water, oil, vanilla, and 1 1/4 cups of the beet purée. Reserve any remaing purée for another use. Pour the batter into the prepared pan. Bake for about 45 minutes, until a toothpick comes out clean. Allow to cool on a wire rack for 20 minutes, then transfer to a dish and allow to cool completely.

Serve with a dollop of creme fraiche and a sprinkle of poppy seeds on each slice.