In a large bowl, cream butter, add icing sugar, lemon juice and zest. Mix 3 cups of the flour with the Lavender food oil and cornstarch.
Beat the ingredients into the creamed mixture a third time. Turn out the dough onto a clean surface and knead in the rest of the flour until soft and smooth.
Line a 9" x 13" metal cake pan with silver foil with enough to hang an inch over the ends. Press dough evenly into pan. Prick all over with a fork. Chill before baking approximately 30 minutes.
Bake at 300degrees F for 40 minutes or until pale gold. While warm, cut into 24 bars. Cool.
*Optional: Before removing from the pan, melt 2-3 oz. white chocolate with 1 tsp. vegetable oil over hot water (double boiler style). The oil thins it to drizzle over the bars.