Preheat oven to 350°F (175°C). Grease and flour a sheet pan.
In a mixing bowl, combine flour and sugar. Set aside.
In a saucepan, melt butter. Add cocoa powder and boiling water. Bring to a boil, then remove from heat.
Pour cocoa mixture over flour mixture. Stir to combine.
Add buttermilk, beaten eggs, baking soda, vanilla extract, and salt. Mix well.
Pour batter into prepared sheet pan. Bake for 20 minutes or until a toothpick inserted in the center comes out clean.
While the cake is baking, prepare the frosting. In a saucepan, melt butter. Add cocoa powder and milk. Bring to a boil, then remove from heat.
Stir in vanilla extract and powdered sugar until smooth. Add chopped pecans.
Pour frosting over warm cake. Spread evenly. Let cool before serving.