1. Preheat oven to 350 degrees F. Grease 13 x 9 x 2-inch baking pan. Dust pan with flour; shake out excess flour.
2. Pour the 1 cup boiling water over the cocoa powder in medium-size bowl; stir to dissolve the cocoa powder. Stir in the buttermilk. Set aside to cool.
3. Sift together flour, baking soda, baking powder and salt onto a piece of waxed paper.
4. Beat together butter and sugar in large bowl at medium speed until light and fluffy. Add eggs, one at a time, beating well after each addition. Stir in vanilla.
5. At low speed, beat flour mixture into butter mixture, alternating with cocoa mixture, beginning and ending with flour mixture. Pour batter into the prepared pan.
6. Bake in preheated 350 degrees F oven for 50 to 55 minutes or until center springs back when lightly pressed with fingertip and a wooden pick inserted in center comes out clean. Cool cake in pan on wire rack.Prepare Glossy Chocolate Frosting:
7. Beat together powdered sugar, evaporated milk and vanilla in a bowl. Melt semisweet chocolate pieces in top of double boiler over hot, not boiling water, or in a microwave oven according to manufacturers instructions. Stir 10X sugar mixture into melted chocolate. Let cool, stirring occasionally, until good spreading consistency.
8. Spread top of cake with frosting. Cut into squares to serve.