Stir together flour, baking powder, and salt. Beat butter and shortening for 30 seconds. Add sugar and beat until fluffy. Add egg, milk, and vanilla, beating well. Add dry ingredients to beaten mixture, blending thoroughly. Cover and chill at least 3 hours. Working with 1/2 the dough at a time, on a lightly floured surface roll to 1/8-inch thickness. Cut into desired shapes. Place on an ungreased cookie sheet. Bake at 375 for 8 minutes or until done.