Heat oven to 350°F. Grease 13 x 9-inch baking pan. Prepare crust: In food processor fitted with chopping blade, combine flour, powdered sugar, and salt. Add butter and vanilla, and process just until combined and mixture comes together. Press dough evenly into baking pan. Bake 25 minutes, or until golden. Cool on wire rack.
In small bowl, combine sugar, flour, and lemon peel. In large bowl, whisk eggs until frothy. Gently beat sugar mixture into eggs until well blended. Gradually stir in lemon juice until combined. Pour over crust and bake 35 minutes, or until top is set. Cool thoroughly on wire rack.
Cut into 24 squares. Cover tops with powdered sugar. Trace a template in the shape of a hen to make stencil, place on top of each square, and sprinkle with blue sugar crystals.