In a large mixing bowl beat butter for about 30 seconds. Add sugar, baking powder, and salt. Beat until combined, scraping sides of bowl occasionally. Beat in egg, milk, and vanilla until combined. Beat in as much of the flour as you can with a mixer, Stir in any remaining flour. Divide dough in half, place in ziploc bag or saran wrap, refrigerate at least one hour or till easy to handle.
On a lightly floured surface, roll out 1/4 of the dough to about 1/8 inch thick. Cut with cookie cutters. Place one inch apart on ungreased baking sheet. Bake one 375° for 7-8 min or till edges are brown and cookie is firm.
ICING: You want the icing to be quite thick so you can paint in on the cookies with a tiny paint brush. I use the smallest foam brush I can find and cut the corners off to make a skinny end. Mix together sifted powdered sugar, vanilla, and only a little milk at a time, and coloring till very well incorporated and no lumps remain. If you icing is too thin, add more sifted powdered sugar. If you add sprinkles, be sure to do it right after painting the cookie while it is still wet so they will stick.
HEART cookies: Use different size cookie cutters. When painting, paint smallest ones first and allow to dry. Then paint larger ones, placing the smaller ones on top while frosting is wet, when it dries it works like a glue.