Begin by making the candied rose petals. To do this, froth the egg white with a fork and gently dip each rose petal into the egg white. Then dip the petal into the granulated sugar and coat both sides. Place them gently a cooling rack in a cool place for 3-4 hours to harden. While you let them rest, you can make the cookies:
Preheat the oven to 400°F.
In a stand mixer, cream together butter and sugar. Then add the egg, ½ tsp of vanilla, and baking powder. Mix until incorporated.
Next, add ½ C of flour and continue beating. Add in the rest of the flour ¼ C at a time until a dough forms. Using your hands, form the dough into a ball and let chill in the fridge or freeze for 5-10 minutes.
Flour your working surface and, once chilled, roll out the dough to about ¼ inch thickness and cut out the cookies. Carefully transfer the cookies onto a greased baking sheet, using a dough scraper if available.
Bake at 400°F for 7 to 10 minutes. Remove them before the edges brown and LET COOL. The cookies will hold their shape once cooled but are especially fragile right out of the oven.
While cooling, make the frosting by combining the confectioner's sugar and milk.
Once the cookies are cool enough to hold their shape, frost each by using a spoon to spread the frosting in a circular motion until it nearly reaches the edge.
Before the frosting solidifies, garnish with the candied rose petals. Eat and enjoy!