• Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 18x18 inch sheet pan. You can also use a 9x13 pan, but the baking time might vary. In that case, start checking for doneness at around 15 minutes.
• In a large mixing bowl, whisk together the flour, granulated sugar, and salt.
• In a saucepan over medium heat, melt 2 sticks of butter. Add 4 heaping tablespoons of cocoa powder and whisk until smooth. Slowly whisk in the boiling water, bring the mixture to a simmer, and then remove from heat.
• Pour the hot chocolate mixture into the dry ingredients and whisk until well combined. Set aside.
• In a separate bowl, combine buttermilk, eggs, baking soda, and vanilla extract.
• Gradually add the wet ingredients to the chocolate mixture, whisking until incorporated.
• Pour batter into the prepared pan and bake for 20-25 minutes, or until a wooden skewer inserted into the center comes out clean or with just a few crumbs attached. The edges should be slightly pulled away from the sides of the pan.

Icing

• While the cake bakes, prepare the icing. In a medium saucepan over medium heat, melt the butter. Whisk in the cocoa powder until smooth. Remove from heat.
• Whisk in milk, vanilla, and powdered sugar (use 2 cups to start, and add more if needed). Stir in chopped pecans.
• Pour the warm icing over the hot cake. Spread it evenly with an offset spatula. The warm cake helps the icing spread evenly. This step might seem counterintuitive, as most cake recipes advise against icing a warm cake. However, it’s what gives this recipe its characteristic fudgy texture.
• Let cool for at least 30 minutes before slicing and serving. This cake is best served warm and will stay fresh for several days if covered tightly with plastic wrap.