Preheat oven to 350°F (175°C) and grease a 9x13-inch baking pan.

In a large bowl, whisk together flour, sugar, and salt.

In a saucepan over medium heat, melt butter, then stir in water and cocoa powder. Bring to a gentle boil and remove from heat.

Pour the cocoa mixture into the dry ingredients and mix well.

In a separate bowl, whisk together sour cream, eggs, baking soda, and vanilla. Add to the batter and mix until smooth.

Pour the batter into the prepared pan and bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.

While the cake bakes, prepare the frosting. In a saucepan, melt butter and mix in cocoa powder and milk. Bring to a gentle simmer, then remove from heat.

Whisk in powdered sugar and vanilla extract until smooth. Stir in pecans if using.

Pour warm frosting over the hot cake and spread evenly.

Let the cake cool before slicing and serving.