Preheat the oven to 350°F. Grease and flour a 9" x 13" pan., To make the cake: Melt the butter; stir in the cocoa and hot water. In a separate bowl, combine the flour, sugar, soda, and salt., Pour the cocoa mixture over the dry ingredients, stirring to blend. Beat in the buttermilk or yogurt, eggs, and vanilla. Pour the batter into the prepared pan., Bake the cake for 30 minutes, or until it tests done. You'll smell the chocolate aroma, and the cake will begin to pull away from the edge of the pan. Remove the cake from the oven and cool it on a rack., To make the frosting: Melt the butter in a medium-sized saucepan. Stir in the cocoa and buttermilk or yogurt., Bring the mixture to a boil, then remove it from the heat and mix in the confectioners' sugar, beating until smooth., Pour the frosting over the cake while the frosting is still warm. Serve the cake right from the pan.