With the rack in the middle position, preheat the oven to 190 °C (375 °F). Lining one to two baking sheets with parchment paper. You'll do about four batches (your sheet filled with cookies x 4 in the oven).
In a bowl, combine the flour, cornstarch, baking powder, and salt. Set the bowl aside.
In another bowl, cream the butter with the sugar and vanilla using an electric mixer for about 3 minutes. Add the eggs, one at a time, and continue beating until smooth. At low speed or with a wooden spoon, combine with the dry ingredients that you have put aside.
With your hands, shape the dough into two discs. Cover them with plastic wrap and put them in the refrigerator for about 1 hour.
On a floured work surface, roll out the dough to about 3 to 5-mm (1/8 to 1/4-inch) thick. Cut out cookies, using a cookie cutter of your choice. Spread the cookies of the same size on the same sheet, because the cooking time may vary.
Bake, one sheet at a time, from 8 to 10 minutes or until the edges are lightly golden brown. Let them cool.
In a bowl, gently beat the egg white and icing sugar with an electric mixer for about 2 minutes or until it forms soft peaks. You can color the icing the color of your choice.
Using a pastry bag fitted with a small plain tip, decorate your cookies.
Do not throw out the leftover dough. You can roll it and make more cookies.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.