In a large bowl with hand mixer or in stand mixer with paddle attachment, beat the butter, brown sugar, and sugar on medium speed until light and fluffy, about 2 minutes.
Beat in the egg and vanilla. Scrape down the sides and bottom of the bowl as needed. In a separate bowl, combine flour, cornstarch, baking soda, salt, and lavender. Add to the wet ingredients and beat on low until combined. Add in the chocolate chunks and mix to combine evenly.
Cover dough with aluminum foil or plastic wrap and chill for at least 1 hour and up to 2 days. Remove cookie dough from the refrigerator and allow to sit at room temperature for 10 minutes. Preheat oven to 350 degrees. Line 2 large baking sheets with parchment paper or silicone baking mats. Set aside. Roll dough into about 1-inch balls using your hands. Space about 2 inches apart on the baking sheets.
Bake for 10 to 12 minutes, until lightly golden brown around the edges. Immediately sprinkle with sea salt once removed from the oven before cooling. Cool for 5 minutes on the baking sheet, then transfer cookies to a cooling rack to cool completely.
Cookies stay fresh covered at room temperature or stored in an airtight container for up to one week.