Preheat oven to 375°F. Grease a 15x10x1-inch rimmed baking pan.
In a large bowl stir together flour, sugar, baking powder, salt, and baking soda.
In a small saucepan combine the 1 cup butter and water; bring to a boil over medium-high heat. Remove from heat and stir into flour mixture.
In a small bowl whisk together eggs, sour cream, and vanilla until combined. Add to flour mixture and whisk to combine.
Transfer batter to prepared pan, spreading evenly.
Bake until golden brown and a toothpick inserted in center comes out clean, about 20 minutes. Cool in pan on a wire rack 20 minutes.
In a large saucepan combine the remaining ½ cup butter and milk. Bring to a boil over medium heat. Remove from heat and gradually whisk in powdered sugar, the remaining ½ tsp. vanilla extract, and a pinch of salt.
Spread over warm cake. Sprinkle with pecans.
