Preheat the oven to 350F and grease a 10x15 inch sheet pan.
Whisk the flour, sugar, baking soda, and salt in a large bowl.
In a saucepan, bring the butter, water, and cocoa powder to a boil, then pour it over the dry ingredients and stir until combined.
Stir in the buttermilk, sour cream, and vanilla until the batter is smooth.
Spread the batter into the pan and bake for 20-24 minutes, until the center springs back when lightly pressed.
During the last few minutes of baking, make the frosting: bring the butter, cocoa powder, and milk to a simmer, then remove from the heat and whisk in the powdered sugar and vanilla until smooth.
Pour the warm frosting over the warm cake and spread it quickly to the edges, then sprinkle with the chopped pecans if using.
Let the cake cool for 1 hour so the frosting sets into its signature fudgy layer, then slice into squares.
