Heat oven to 350°F. Grease a 13x18-inch rimmed sheet pan and line with parchment.
Whisk flour, sugar, salt, and baking soda in a large bowl.
Melt butter with water and cocoa in a saucepan over medium heat. Bring to a gentle bubble and whisk smooth.
Pour the hot cocoa mixture into the dry ingredients and stir until mostly combined.
Beat in eggs, buttermilk, and vanilla until the batter is smooth and pourable.
Spread batter evenly in the prepared pan. Bake 16–20 minutes until a toothpick comes out clean.
Start the frosting near the end of baking: simmer butter, milk, and cocoa in a saucepan, whisking until smooth. Remove from heat.
Whisk in powdered sugar and vanilla until glossy. Fold in pecans.
Pour warm frosting over the hot cake and spread quickly. Let set 20 minutes before slicing.