In a small bowl, beat butter and sugar until crumbly, about 2 minutes. Beat in egg and vanilla. Combine the flour, baking powder, and salt; add to egg mixture alternately with milk. Pour into a 5 3/4x3x2 inch loaf pan coated with cooking spray. Bake at 340 degrees for 45-50 minutes or until a toothpick inserted near the center comes out clean. Cool for 10 minutes before removing from pan to a wire rack to cool completely.