1. In a large bowl, sift together flour, salt, and baking powder. Set aside.

2. In the bowl of an electric mixer fitted with the paddle attachment, beat together the butter and sugar on medium speed until fluffy, about 3 minutes. Add egg, and beat until smooth, about 1 minute.

3. Add reserved flour mixture, and mix on low speed until thoroughly combined. Stir in vanilla. Wrap dough in plastic, and refrigerate for about 45 minutes.

4. Make cookie pattern and heart stencil while wrapped dough is chilling. For the pattern, have your child trace his or her hand on a manila file folder, and cut out the tracing. You will lay this on the dough and trace around it with the tip of a paring knife. For the heart stencil, cut a heart shape from a piece of parchment paper, discarding center.

5. Preheat oven to 325 degrees. Line 2 baking sheets with parchment paper. On a well-floured work surface, roll out cold dough to 1/8 inch thick. Cut out hand shapes. Transfer cookies to prepared baking sheets. Refrigerate until firm, about 15 minutes. Dough scraps may be re-rolled twice.

6. To decorate the cookies with sanding sugar hearts, lay the heart stencil over a cookie. Combine the egg white with the water in a small bowl. Brush the egg wash in the heart, and sprinkle with sanding sugar. Lift stencil and repeat with remaining cookies. Bake until edges just begin to brown lightly, 8 to 10 minutes. Let cookies cool on pans for 3 minutes before transferring to wire racks to cool completely.