Cream butter in a large mixing bowl; gradually add sugar, beating until light and fluffy. Add egg, milk, and vanilla, and beat well.
Combine flour, baking powder, salt, and oats, stirring well. Add to creamed mixture; stir well. Divide dough in half, and chill thoroughly.
Roll half of dough to 1/2-inch thickness on a lightly floured surface; keep remaining dough chilled until ready to use. Cut with a heart-shaped cutter. Place on lightly greased cookie sheets. Bake at 375° for 8 to 10 minutes. Remove from cookie sheets, and cool on wire racks. Repeat procedure with remaining dough. Frost with Powdered Sugar Frosting.