Sift together flour, baking powder, and salt. Set aside. Place butter & sugar in large bowl of electric stand mixer and beat until light in color. Add egg and milk and and beat to combine. Put mixer on low speed, gradually add flour, and beat until mixture pulls away from side of bowl. Divide the dough in half. Shape each half into a disk 3/4 to 1 inch thick, wrap in wax paper, and refrigerate for 2 hours. 

Line two baking sheets with parchment paper. 

On a lightly floured large work surface, roll out one dough disk 1/8 inch thick. Using the 7 different-sized cookie cutters, cut out 2 cookies of each size, for a total of fourteen cookies. Transfer them to the prepared baking sheets, putting the larger stars together and the smaller stars on the second sheet. Roll and cut out the second dough disk the same way and add the cookies to the sheets. You should have 28 cookies total, four stars of each size. (If needed, gather up the dough scraps, reroll, and cut out any missing stars.) Refridgerate the baking sheets for 15 minutes. 

Position two oven racks, evenly spaced, in the oven and preheat the oven to 375 degrees. 

Bake the cookies for 12 to 15 minutes, until they look firm, switching the baking sheets between the racks about halfway through the baking time. Keep an eye on the smaller stars, as they will have to come out of the oven sooner. Transfer the baking sheets to wire racks and let cool for 5 minutes, then carefully transfer the cookies to the racks and let cool completely. 

Fit the pastry bag with the pastry tip and fill with white icing. Outline the points of the stars by piping small dots along their edge, stopping about 1/3 away from the tip. Let the stars sit for about 30 minutes, until completely dry. 

For each tree, have ready 14 cookie stars, two of each size. Place the largest star on a plate and pipe a small amount of icing in the center of the star. Place the same-sized star on top, angling it slightly so the the star tips don't line up. Continue topping the stack with stars in decreasing size, always adding 2 same-sized stars at a time, adhering them with a bit of icing, and angling them slightly. End with one of the smallest stars on top lying flat, and then place the other smallest star so that it stands upright, adhering it in place with a little icing. Let dry completely before displaying.