1.Preheat oven to moderately slow, 160°C. Grease and line a 20cm square cake pan.
2.In a saucepan, combine butter, chocolate, sugar and milk. Stir over low heat, without boiling, until smooth.
3.Sift flours together and blend into chocolate mixture with eggs and vanilla.
4.Pour into pan. Bake 1 hour 10 minutes, until cooked when tested.
5.Cool in pan 10 minutes. Turn onto a wire rack to cool completely. Cut in half horizontally and fill with cream.
6.To make the hot fudge sauce; in a saucepan, combine chocolate, cream, sugar and vanilla. Heat on low, 3-4 minutes, stirring, until melted. Keep warm. Spoon sauce over cake to serve.