2 cups all-purpose flour

2 cups granulated sugar

1/4 teaspoon salt

1 teaspoon baking soda

1/2 cup buttermilk

2 large eggs

1 teaspoon vanilla extract

1 cup unsalted butter (2 sticks)

1/3 cup unsweetened cocoa powder

1 cup water

1/2 cup unsalted butter (for frosting)

1/4 cup unsweetened cocoa powder (for frosting)

1/4 cup buttermilk (for frosting)

3 to 4 cups powdered sugar

1 teaspoon vanilla extract (for frosting)

1 cup chopped pecans (optional)

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Preheat oven to 350°F (175°C). Lightly grease a 10×15-inch jelly roll pan or rimmed baking sheet.
In a large bowl, mix together flour, sugar, salt, and baking soda. Set aside.
In a medium saucepan, melt 1 cup butter over medium heat. Stir in 1/3 cup cocoa powder and 1 cup water. Bring to a light boil, stirring constantly. Remove from heat.
Pour the hot mixture into the dry ingredients and stir until combined.
In a small bowl, whisk together 1/2 cup buttermilk, eggs, and 1 teaspoon vanilla extract. Add this to the batter and stir until smooth.
Pour batter into prepared pan and spread evenly. Bake for 20–25 minutes, or until a toothpick inserted in the center comes out clean.
While the cake bakes, make the frosting: In a saucepan, melt 1/2 cup butter, then stir in 1/4 cup cocoa powder and 1/4 cup buttermilk. Heat until just bubbling, then remove from heat. Quickly whisk in powdered sugar and vanilla until smooth. Add chopped pecans if desired.
Pour warm frosting over hot cake as soon as it comes out of the oven, spreading evenly. Let frosting soak in.
Allow cake to cool at least 30 minutes before slicing. Serve warm or at room temperature.

Add a teaspoon of instant coffee to the cake batter for a deeper chocolate flavor.

Sprinkle chocolate chips over the batter before baking for extra chocolatey goodness.

Swap pecans for walnuts or leave nuts out entirely for a nut-free version.

Use dark cocoa powder for a richer, darker chocolate flavor.

Frost with cream cheese frosting instead of chocolate for a tangy twist.

Store leftover cake covered at room temperature for up to 3 days or refrigerate up to 5 days.

Freeze unfrosted cake for up to 2 months; thaw and frost before serving.