Preheat oven to 350°. Coat a 9- by 13-in. baking pan with cooking-oil spray and set aside.
In a medium saucepan, bring raisins and 3 cups water to a boil. Reduce heat to maintain a steady simmer and cook until very tender, about 15 minutes. Remove from heat, add butter, and let sit until butter melts.
Meanwhile, in a large bowl, combine flours, sugar, cocoa powder, baking soda, cinnamon, cloves, allspice, and salt. Set aside.
Stir raisin-butter mixture into flour mixture. Transfer batter to prepared pan and bake until a toothpick or knife inserted in the center comes out clean, about 45 minutes. Serve cake warm or at room temperature.