You won't taste the pureed beets, but they make this cake extra moist and fudgy, even with the addition of whole-grain spelt flour. The beets also play up the cake's full-bodied, not-too-sweet flavor.
For the chocolate ganache frosting:
Put chocolate in a bowl. In a small saucepan, heat cream until just simmering. Pour cream over chocolate and let stand 10 minutes; then stir until smooth. Let stand, stirring occasionally, until thickened and spreadable, about 1 hour.
For the cake:
Place beets in a steamer basket set in a saucepan filled with 2 inches of simmering water. Cover and steam until beets are tender when pierced with a sharp knife, about 25 minutes. Let cool slightly, then puree in a food processor until smooth.
Preheat oven to 350°F. Oil one 9-inch square or round cake pan. Line bottom with a parchment.
In a large bowl, whisk together both flours, sugar, cocoa powder, baking soda, and salt. Whisk in eggs, water, oil, vanilla, and 1¼ cups beet puree (reserve remaining puree for another use).
Pour batter into prepared pan. Bake, rotating pan halfway through, until cake is set and a cake tester inserted in center comes out clean, about 35 minutes.
Transfer pan to a wire rack and let cake cool, 20 minutes. Invert cake onto rack and let cool completely.
Place cake on a cake plate and spread evenly with chocolate ganache. (Cake can be kept, covered, up to 2 days.)
Courtesy of "A New Way to Bake" by Martha Stewart