1. Preheat the oven to 400°F. Butter a 9-inch round spring-form pan. In a medium bowl, combine the flour, sugar, and baking soda. Stir well to blend.

2. In a medium (3-quart) nonstick saucepan, combine the water, butter, cocoa powder, and espresso, if desired, and stir to blend. Cook over medium-high heat, stirring constantly, until well blended and it just comes to a boil, about 2 minutes. Gradually stir in the dry ingredients until blended. Reduce heat to medium. In a small bowl, whisk the buttermilk, egg, and vanilla together. Add to the batter and mix vigorously for about 1 minute, or until well blended. Remove from heat and pour into the prepared pan.

3. Bake for about 15 minutes, or until the top just begins to crack and the center is still slightly moist when pierced with a skewer. Remove from the oven.

4. While the cake is baking, make the frosting: Rinse out the pan used to make the batter. Add the butter, cocoa powder, and buttermilk to the pan and bring to a boil over medium-high heat, stirring constantly until blended. Add the confectioners’ sugar, vanilla, and walnuts and mix to combine. Remove from heat.

5. While the cake is still hot, pierce holes allover the top with a skewer and pour the frosting over. Keep piercing the cake after you have poured over the frosting to create a fudgy interior.

6. Transfer the cake to a platter and release and remove the sides of the springform pan, allowing the frosting to drip down the sides of the cake. Let sit for at least 15 minutes, or until the frosting is set. Serve warm or at room temperature.

Advance Preparation:

May be prepared 1 day ahead, covered, and kept at room temperature.

The clever cook could:

1. Double the recipe and use a 9-by-13-inch baking pan. Cut into squares and serve with ice cream.