Grease a 12 cup bundt pan and preheat oven at 350 degrees.
In a large bowl, mix together the flour, cocoa, baking soda, baking powder, salt, cinnamon and nutmeg. Reserve. In a medium bowl, whisk together the oil, brown sugar, eggs, corn sirup and vanilla until smooth and creamy. Add the carrots and pinapple (not drained)while stiring. Add the dry ingredients to the carrot mixture and stir until evenly moistened. Pour batter in pan. Bake until a toothpick inserted in the middle comes out clean (approx. 60-65 minutes). Let cool in pan for 10 minutes. Remove from pan onto wire rack and let cool completely.
Drizzle chocolate icing over cake and let drip down the sides. Sprinkle with chopped nuts.
Chocolate icing: sift the confectionnar's sugar and cocoa together in a small bowl. Slowly add 2-3 tbsp of boiling water while stiring. The icing should be a bit runny since it will be drizzled on the cake.