Preheat the oven to 160ºC . Melt the chocolate with the butter in the top of the double boiler over hot water until smooth. Cool slightly. Beat the eggs with the sugar in a large bowl until light. Slowly beat in the chocolate mixture. Stir in the vanilla. Sift in the flour, 2 Tablespoon at a time, mixing well after each addition. Stir in the salt, sour cream and walnuts. Pour the batter into a greased 23 cm square cake pan. Bake until a toothpick inserted in the centre comes out fairly clean, about 25 to 30 minutes. The centre should be slightly cakey. Cool completely on a wire rack before cutting into bars. MAKES: ABOUT 20 BROWNIES