1. Heat oven to 400 degrees F. Grease 9-inch springform pan. In bowl, mix flour, sugar and baking soda.
2. In medium-size nonstick saucepan, combine water, butter, cocoa powder and espresso powder, if using. Bring mixture just to a boil, stirring constantly, over medium-high heat, about 2 minutes. Gradually stir in flour mixture until blended. Reduce heat to medium. In small bowl, whisk together buttermilk, egg and vanilla. Add to cocoa mixture in saucepan; stir vigorously for 1 minute or until well blended. Pour batter into prepared pan.
3. Bake in 400 degrees F oven 15 minutes or until top just begins to crack and center is still slightly moist when pierced with skewer. Transfer pan to wire rack.Frosting:
4. In medium-size saucepan, combine butter, cocoa powder and buttermilk. Over medium-high heat, bring to a boil, stirring constantly, until blended. Stir in confectioners sugar, vanilla and walnuts; remove from heat.
5. While cake is still hot, pierce holes in top with skewer. Pour frosting over top. Keep piercing cake with skewer to create fudgy interior.
6. Transfer cake to serving plate. Remove sides of pan, letting frosting drip down sides of cake. Let stand until frosting is set, 15 minutes. Serve warm or at room temperature. (Can be prepared a day ahead; store at room temperature.)