Transfer raisins and water to the beaker of a hand-held mixer or blender and puree. Stir raisin and beet puree, dissolved cornstarch, sugar, and vanilla into cocoa mixture. Before proceding, this mixture should be at room temperature. Quickly stir dry ingredients into cocoa mixture. Do not stir longer than needed to thouroughly mix the two.
Scrape batter into prepared pan and smooth the surface.
Bake on top shelf for 35 to 40 minutes, or until a toothpick inderted in the middle of the cake comes out clean. Let cake sit in pan for 10 to 15 minutes and then remove to a wire rack. Cool cake completely before cutting.