Preheat oven to 350 degrees F. Line baking sheet with parchment paper. In a bowl, combine lavender sugar and lemon zest. Rub together with fingers for about 1 minute. Combine sugar/lemon mixture with butter and beat on high with an electric mixer for 1 minute, then add lemon juice, egg, and vanilla. Beat until combined. Add flour, baking soda, and salt, and beat until combined. Roll into tablespoon-size balls, then cover with lavender sugar. Bake for 10 to 12 minutes.