Preheat oven to 350 degrees F. Coat the inside of a rimmed half sheet baking pan with non-stick cooking spray that has flour in it. Set aside.
Mix flour, sugar, baking soda, and salt in a large bowl. Set aside.
Combine water, cocoa powder, and butter in a medium saucepan. Bring to a boil, whisking often. Remove from heat, pour into the bowl with dry ingredients. Mix together until just combined.
Add buttermilk and eggs and mix to combine. The batter will be thin.
Pour batter into prepared baking sheet. Make sure it's distributed evenly.
Bake for 15-to 20 minutes until a toothpick inserted in the center comes out clean and the cake bounces back when lightly touched.
Remove from oven and place baking sheet on a wire rack to cool to room temperature.
In a medium saucepan, heat over medium heat buttermilk, butter and cocoa until bubbly. Mix in sugar and vanilla until smooth. Add in nuts and blend until incorporated.
Spread frosting over the entire cake, spreading into an even layer. Let frosting set before cutting the cake into 15 slices. Store any leftovers in an air-tight container or cover the baking sheet with plastic wrap.
If you don't have buttermilk on hand, you can make sour milk instead. Add 1 teaspoon white vinegar to 1 cup milk and let it sit for a few minutes. Then use as directed.