Preheat the oven to 170ºC/gas 3 and grease and line a 12cm x 25cm loaf tin.
In a bowl, sift together the flour, cocoa, baking powder, bicarbonate of soda and cinnamon. Add a good grating of nutmeg, and set aside.
Purée the cooked pumpkin or squash, then whisk with the buttermilk in a jug. Melt and add the chocolate, followed by the vanilla extract. Set aside.
Using an electric whisk, cream the butter and both sugars in a separate bowl until fluffy. Gradually beat in the eggs, whisking continuously, then fold in half the flour until combined, followed by half the buttermilk liquid. Repeat until everything is mixed in.
Pour the batter into the loaf tin, then bake it in the oven for 1 hour, or until golden and a skewer inserted comes out clean. Remove it from the oven and cool in the tin for 10 minutes, then take it out and cool on a wire rack.
While the cake is cooling, make the icing. Peel and finely grate the ginger, then place in a bowl. Finely grate the clementine zest and put aside. Squeeze the clementine juice into the bowl, then beat in the icing sugar to make a thick but spoonable topping. Pour over the warm cake, sprinkle on the zest and serve.