1. Preheat oven to 350°F (175°C). Grease a 13×18 inch sheet pan.

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

3. In a medium saucepan, melt the butter over medium heat. Stir in cocoa powder and boiling water. Remove from heat once smooth.

4. In a separate bowl, whisk eggs, buttermilk, and vanilla.

5. Pour cocoa mixture into the dry ingredients, stir well, then add egg mixture. Mix until fully combined.

6. Pour batter into prepared pan and bake for 18–22 minutes, or until a toothpick comes out clean.

7. While the cake bakes, make the frosting. In a saucepan, melt butter. Stir in cocoa and milk. Once smooth, remove from heat.

8. Whisk in powdered sugar and vanilla until the frosting is glossy and pourable.

9. While cake is still warm, pour frosting evenly over the top and spread gently. Add chopped pecans if desired.

10. Let cool completely before slicing and serving.