Break marzipan into small pieces in a mixing bowl. Sift the powdered sugar over the marzipan and add the ground almonds, flour, rose water and egg white. Use your hands to combine the ingredients in to a dough. The dough will be somewhat sticky but it will get firm after it's sat in the fridge. If too sticky, add some more ground almonds and powdered sugar.
Wrap the cookie dough in plastic wrap and refrigerate for at least an hour.
Roll the dough into small balls, no more than 1-inch in diameter. Sprinkle your hands with a little flour or powdered sugar if the dough is too sticky. Place the balls on a lined cookie sheet and press 3 almond halves evenly around each cookie, pointed side up. Brush each cookie with the egg yolk mixture and bake for about 15 minutes until the egg wash turns golden in color.
Transfer the cookies to a wire rack to cool completely. Store in an airtight container for up to several weeks.
Not all marzipan is created equal. Some use a higher ratio of sugar to almonds which may result in your cookies deflating. For the best results, be sure to use the homemade marzipan.