Preheat the oven to 160 degrees C. Grease and flour a 22x33cm cake tin.
Sift together the flour, bicarb soda and salt.
Melt the chocolate in a double boiler or the microwave. Let cool until just warm.
In the bowl of an electric mixer, beat the butter until creamy. Add sugar and beat until light and fluffy, about 3 minutes. Add the eggs one at a time and beat. Add vanilla. Add chocolate and mix well. Add flour, alternately with sour cream. Gradually add hot water until blended. Pour into cake tin.