Preheat oven to 350°F. For the Cake, mix flour, baking powder and salt in medium bowl. Set aside.
Beat butter in large bowl with electric mixer on medium speed 30 seconds or until softened. Add granulated sugar and vanilla; beat until light and fluffy, scraping down sides of bowl frequently. Beat in eggs, 1 at a time. Alternately beat in flour mixture and milk on medium-low speed just until mixed. Pour into greased and floured 13x9-inch baking pan.
Bake 35 to 40 minutes or until toothpick inserted in center comes out clean. Cool completely on wire rack.
For the Frosting, beat butter and vanilla in large bowl until light and fluffy. Gradually add confectioners' sugar, beating well after each addition and scraping sides and bottom of bowl frequently. Add milk; beat until light and fluffy. Frost top of cake with frosting.