Special equipment: 6 cup fluted tube pan sprayed with vegetable oil spray
Preheat oven to 350 degrees. Position rack in lower third of the oven. On low speed with an electric mixer, mix the sugar, flour, cocoa, baking powder, baking soda, and salt until thoroughly combined.
Add the softened butter, whole egg, and egg whites. Set a timer for 2 minutes as you begin beating on medium speed. As soon as the dry ingredients are moistened, increase the speed to high and continue mixing until the 2 minutes are up. Stir the optional espresso powder into the yogurt, if you are using it. Add the yogurt and vanilla to the batter. Beat at high speed for exactly 2 more minutes.
Scrape the batter into the pan. Bake 30 to 35 minutes or just until a thin wooden skewer inserted in the center of the cake comes out clean. Do not over bake. Cool in the pan on a wire rack for ten minutes before unmolding on the rack to cool completely. Serve sprinkled with powdered sugar.
Make the Glaze: In a small heavy bottomed saucepan, combine the brown sugar with the cocoa. Use a wooden spoon to stir in just enough milk to form a smooth paste. Stir in the remaining milk. Cook over medium heat until mixture simmers, and finally begins to boil, stirring constantly and scraping the sides and bottom edges of the saucepan religiously. Boil gently for 1 1/2 minutes, stirring constantly to prevent burning. Off heat, stir in vanilla. Cool to room temperature. Spoon glaze over cake.