Preheat oven to 350°. Grease a half sheet foil cake pan (17x12x1 in.).
In a bowl, cream butter and sugar until light and fluffy, 5-7 minutes. Add vanilla and eggs, 1 at at time, beating well after each addition. In another bowl, whisk flour, baking powder and salt; beat into creamed mixture alternately with milk. Transfer to prepared pan.
Bake until a toothpick inserted in center comes out clean, 25-30 minutes. Place on a wire rack; cool completely.
For frosting, beat cream cheese and butter. Add the confectioners' sugar, milk and vanilla. Reserve 1/4 cup of frosting for field markings. Tint remaining frosting green; spread over top of cake. Pipe white yard lines and numbers accordingly. Decorate field with goal posts and gumdrops for football players.