Grease or spray a 9x9x2 baking pan. Preheat oven to 350. Cream butter and sugar. Add egg, milk, and vanilla; beat well. Stir together flour, baking powder, baking soda, and salt. Add to creamed mixture; beat well. Stir in chocolate chips. Spread in prepared pan. Bake at 350 for 30-35 minutes. Cool; cut into bars.