In a medium bowl whisk the flour.
In a large bowl use an electric mixer, cream the butter and sugar, add the egg yolks 1 at a time, beating until fully blended after the addition of each one. Add vanilla and beat for 1 minute.
Stir in the flour until its just combined. Stir in lavender blossoms and lemon zest. Beat dough until creamy, 1-2 minutes.
Form into a ball and divide into 4 pieces. Flatten and wrap in plastic, refrigerate overnight.
Pre-heat oven to 350 degrees F. Place parchment paper on 2 cookie sheets.
Using a rolling pin roll out each piece of dough separately. Roll on a lightly floured board to a thickness of about 1/8", cut with a 2-2 1/2" round cookie cutter. Place on parchment paper about 1" apart. Gather all the scraps and roll out again to cut more cookies, repeat until all dough is cut into circles.
Bake for 7-9 minutes until lightly golden, let cool completely on sheets. Spread blueberry jam on half of the cookies and top with the flat side of the remaining cookies to form a sandwich.
In a medium bowl whisk together the powdered sugar, lavender syrup and water to blend. Add 1/2 more water at a time if necessary, until desired spreadable consistency. Spread a thin layer atop each cookie sandwich. Top each cookie with a mint leaf while icing is wet. Refrigerate and let sit for a hour until icing is set. Enjoy!
