Preheat oven to 350 degrees F / 180 degrees Grease and flour very well a 8 inch bundt cake pan.*
In a medium bowl combine the two kinds of flour with the baking soda.
In a large bowl cream butter and sugar with an electric mixer. Add eggs and vanilla and beat until smooth. Scrape out any mixture stuck on the sides of the bowl to make sure everything is mixed very well. Add yogurt and beat shortly until absorbed.
Switch off the mixer and add the grated vegetables. Mix gently with a rubber spatula until evenly distributed in the batter. At the end fold in the dry ingredients.
Transfer the batter to the prepared pan. Bake for about 35-40 minutes or until a toothpick inserted in the cake comes out clean. Remove from the oven and let cool completely.
Gently run a butter knife around the edges before inverting the cake. Pour the glaze on top just before serving.
Store for 3-4 days in the airtight container in the fridge.
Melt the chocolate with the water in a double boiler (in a heatproof bowl over a pan of simmering water).
To easily grease and flour the bundt pan: put 1-2 tbs vegetable oil and 1/2 tbs flour in the pan. Using a kitchen brush mix the two ingredients until smooth. Brush very well the pan with the mixture, passing through all the cavities. Add more oil/ flour if necessary.