In a large mixing bowl, cream together the butter, salt, sugar and vanilla until light and fluffy.
Add the lemon zest.
Add the flour and mix until well combined
Roll out the dough between 2 sheets of lightly floured parchment to 1/4 inch thickness. Make sure you're using enough flour or your dough will stick.
Cut out the cookies using a 3 inch circle cookie cutter or any other shapes as preferred.
Transfer the cookies to a parchment-lined baking sheet.
Cover the baking sheet with plastic wrap and place in the freezer. Freeze until firm, about 15 minutes.
Preheat the oven to 160 degrees C. Brush each cookie with beaten egg white and gently push the flowers into the cookies.
Bake the cookies on the middle rack of the oven for about 15-20 minutes or until golden on the edges.
Cool on the baking sheet for 5 minutes, then transfer to a wire cooling rack to cool completely.
Cookies can be stored in an airtight container for up to 5 days.