In medium bowl, mix flour, drink mix, baking powder and 1/2 teaspoon salt; set aside. In large bowl, beat 1 cup butter with electric mixer on medium speed 30 seconds. Gradually add granulated sugar, about 1/4 cup at a time, beating well after each addition. Beat 2 minutes longer. Add eggs, one at a time, beating well after each addition. Beat in 1 teaspoon vanilla. On low speed, alternately add flour mixture, about a third at a time, and watermelon puree, about half at a time, beating just until blended.