Preheat oven to 180 degrees C. Grease and flour a 23cm ring tin. In small saucepan, heat butter, chocolate and water until chocolate melts and mixture is smooth, stirring constantly. Set aside to cool.
Sift together into a large bowl the flour, sugar and bicarb soda. In a small bowl, beat egg slightly and combine with sour cream and vanilla. Stir into flour mixture. Pour in the cooled chocolate mixture. Beat at low speed for 5 minutes until completely combined and the consistency of heavy cream.
Pour batter into prepared ring tin. Bake in the preheated oven for 45 to 60 minutes, or until a toothpick inserted into the centre of the cake comes out clean. Let cool in pan for 10 minutes, then turn out onto a wire rack and cool completely.