Mix chocolate with milk first.Add yogurt and mix
pour this mixture over the base on a tray filled with water and bake for 20 minutes.
Take out of oven and let it cool in normal temperature
Melt chocolate for the topping and mix with cream and beat to make a smooth chocolate sauce. If its too thick ,You can add little milk and heat again in microwave. the chocolate sauce should be of pouring consistency.
Pour on the cake and keep in refrigerator to chill for at least 3 to 4 hours before serving.