1. Preheat oven to 350°F and grease a 10×15-inch jelly roll pan or rimmed sheet pan.

2. In a large bowl, whisk together flour, sugar, baking soda, and salt.

3. In a saucepan, bring butter, cocoa powder, and water to a boil while stirring, then remove from heat.

4. Pour the cocoa mixture into the dry ingredients and mix until combined.

5. Add buttermilk, eggs, and vanilla. Stir until smooth and fully blended.

6. Pour the batter into the greased pan and bake for 18–20 minutes, until a toothpick comes out clean.

7. While the cake bakes, make the frosting: melt butter with cocoa in a saucepan, add milk, and bring to a low simmer.

8. Remove from heat, whisk in powdered sugar and vanilla until smooth. Stir in pecans if using.

9. Immediately pour warm frosting over hot cake and spread evenly.

10. Allow cake to cool and frosting to set before slicing and serving.