Preheat oven to 350°. Grease a 13x9-in. baking pan. In a large bowl, whisk the first 6 ingredients. Stir in eggs, oil and buttermilk. Add water; stir until combined.
Transfer batter to prepared pan. Bake 35-40 minutes or until a toothpick inserted in center comes out clean. Cool completely in pan on a wire rack. Frost cake. If desired, decorate with sprinkles.
Chocolate Buttercream Frosting: In a large bowl, beat 1/2 cup softened butter until creamy. Beat in 2 cups confectioners' sugar, 1/4 cup baking cocoa, 1-1/2 teaspoons vanilla extract and 3-4 tablespoons 2% milk to achieve desired consistency.