For the cake:
2 cups (8.8 ounces) all-purpose flour
2 cups (14 ounces) granulated sugar
1/2 teaspoon kosher salt
1 cup (8 ounces) unsalted butter
1 cup water
1/4 cup (1 ounce) natural unsweetened cocoa powder
1/2 cup buttermilk
2 large eggs
2 teaspoons vanilla extract
1/2 teaspoon baking soda
For the frosting:
3/4 cup (6 ounces) unsalted butter
5 tablespoons natural cocoa powder
1/2 cup milk, whole or 2 percent
1 teaspoon vanilla extract
Pinch of kosher salt
3 cups (15 ounces) powdered sugar, sifted
1 cup chopped pecans (4.5 ounces)
Preheat the oven and prep the pan:
Preheat the oven to 350°F. Spray or butter a half-sheet pan (13x18 inches) and line the bottom with parchment.
Combine the dry ingredients:
Whisk the flour, sugar, and salt together in a large mixing bowl.
Make the batter:
In a saucepan over medium-low heat, combine the butter, water, and cocoa powder. Whisk until melted and smooth, and then bring to a simmer. Once simmering, remove from the heat.
Using a whisk or electric mixer, beat the hot cocoa mixture into the dry ingredients until combined. Beat in the buttermilk, eggs, and vanilla, followed by the baking soda, until combined.
Bake the cake:
Spread the batter into the prepared pan and bake for 25 to 28 minutes, or until a toothpick inserted in the center comes out clean.
Remove from the oven and set the pan on a wire rack to cool briefly, about 5 to 10 minutes, while you make the frosting.
Frost the cake:
As soon as the cake is done baking and on the cooling rack, combine the butter, cocoa, and milk for the frosting together in a saucepan and bring to a boil. Remove from the heat and stir in the salt and vanilla. Beat in the powdered sugar with an electric mixer or by hand.
Pour the hot frosting over the still-warm cake and spread evenly. Sprinkle the top with chopped pecans or other topping.
Allow to set, then serve immediately or cool completely and cover to serve later. You can also wrap the cake well in foil or plastic wrap and freeze.