Preheat oven to 375°. Grease a 15x10x1-in. baking or jellyroll pan.
In a large bowl, whisk the first five ingredients (flour, sugar, baking powder, salt, and baking soda).
In a small saucepan, combine butter and water; bring just to a boil. Whisk into the flour mixture.
In a small bowl, whisk eggs, sour cream and extract until blended; add to flour mixture, whisking constantly.
Pour batter into a prepared pan. Bake until golden brown, 20 minutes. Cool on a wire rack 20 minutes.
For frosting, combine butter and milk in a large saucepan; bring just to a boil. Remove from heat; gradually stir in powdered sugar and extract.
While the frosting is warm, spread over the warm cake. Allow to sit for 1 hour before serving.