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1
Preheat oven to 400; butter a round 9-inch round springform pan.
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2
In a bowl, combine the flour, sugar, and baking soda; stir to blend.
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3
In a medium (3-quart) nonstick saucepan, combine the water, butter, cocoa powder, and espresso; stir to blend.
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4
Cook over med-high heat, stirring constantly, until well blended and it just comes to a boil, about 2 minutes.
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5
Gradually stir in the dry ingredients until blended.
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6
Decrease heat to medium.
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7
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.
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8
Remove from heat and pour into the prepared pan.
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9
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.
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10
While the cake is baking, make the frosting-add the butter, cocoa powder, and buttermilk to the pan and bring to a boil over med-high heat, stirring constantly until blended.
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11
Add the powdered sugar, vanilla, and walnuts and mix to combine.
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12
Remove from heat.
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13
While the cake is still hot, pierce holes all over the top with a skewer and pour the frosting over.
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14
Keep piercing the cake after you have poured over the frosting to create a fudgy interior.
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15
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.
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16
Let sit for at least 15 minutes or until the frosting is set.
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17
Serve warm or at room temperature.
