In another bowl, combine butter and 1/2 cup sugar; beat until light and creamy. Add beaten egg a little at a time, beating until smooth between each addition. Beat in the cocoa mixture. Combine cake flour, baking soda, and salt. Resift together and beat into the butter/cocoa mixture in two parts alternating with 1/4 cup buttermilk. Pour batter into prepared pan and bate at 375 degrees for 30 35 minutes. Let cake rest a few minutes before turning out onto a cake rack to cool. For a glaze: In a double boiler, melt chocolate, cream, and vanilla. Stir until smooth and let cool to warm. Pour over cake. If desire, before glazing, split cake in half and spread your choice of filling between the layers. From Williams-Sonoma's April, 1991 Catalogue, page 7.