Cream butter, sugars, egg in the bowl of a stand mixer fitted with a paddle attachment. Beat on medium high speed until the mixture is light and fluffy (about 3-4 minutes).
Scrape down the sides of the bowl. Add lemon zest, honey, and lavender. Beat for another 2 minutes until combined.
Add the flour, cornstarch, and a pinch of salt directly to the bowl of your stand mixer (no need to whisk separately). Add baking soda to the top of the pile and add lemon juice directly to the baking soda pile. It will bubble and foam- this is good! It means your baking soda is activated. If it doesn't bubble, it is most likely old and expired. Beat for 1-2 minutes, until combined. Scrape sides of the bowl as necessary.
Cover bowl with plastic wrap and transfer to the refrigerator to chill for at least 2 hours.
Meanwhile, whisk together the sugar and lavender. You will roll your cookies in this before baking.
Towards the end of chilling time, preheat your oven to 350 degrees F and line a baking sheet with parchment paper.
When the dough is properly chilled, take a couple tablespoons of dough and roll into a ball. Dip the ball to into the bowl with the lavender sugar and roll it around until it is evenly coated. Transfer ball to your prepared baking sheet and press down slightly to flatten a bit. Repeat with remaining dough.
Bake for 10 minutes or until the edges are slightly brown.
If you are making these cookies for a cookie sandwich (or if you are a perfectionist), I highly recommend the "cookie scoot" to get perfectly round cookies. Take a round cookie cutter (larger than the size of the baked cookie) and place it over the hot cookies right out of the oven. Keeping the cookie cutter touching the baking sheet, quickly move it around in a circular motion. This will make the edges of the cookie touch the sides of the cookie cutter and after a few turns, it will be a perfect circular shape!
Allow the cookies to cool on the baking pan for a few minutes and then transfer to a wire rack to cool completely. Enjoy!