In the bowl of a stand mixer fitted with a paddle attachment cream the butter, brown sugar, and granulated sugar on medium speed for 2 minutes. Scrape down the sides of the bowl with a rubber spatula. Add the vanilla extract and large egg and mix again to combine.
Add the flour, cornstarch, baking powder, baking soda, and kosher salt. Use a fork or whisk to combine the top dry ingredients together. Then close the stand mixer and turn the mixer on low and mix until the dry ingredients are partially combined.
Add 1 ¼ cups Heath toffee bits and ¼ cup mini chocolate chips and mix again until combined and evenly distributed. Cover the dough bowl with plastic wrap and chill in the refrigerator for 45 minutes. This step allows the dough to firm up just a tad (but not so much that it’s difficult to scoop)… also prevents overspreading so I encourage you not to skip this step.
20 minutes prior to pulling the dough out of the refrigerator, preheat your oven to 375° F.
When ready to bake use a 2 T cookie scoop (I used a #30) and roll into a ball. Dip the tops of each cookie into the remaining ½ cup of Heath toffee bits. Place each dough ball on a parchment lined baking sheet (toffee bit side up) keeping about 2 inches between each cookie. Bake 9-10 mins. Or until edges are set.
Remove from oven, top with flaky sea salt (if desired) and let cool on pan for 5-10 minutes then transfer to a wire rack to cool completely.
Notes
Store leftover cookies in an airtight container at room temperature for 3-4 days. To freeze, allow cookies to cool completely before wrapping individual cookies in plastic wrap and then placing in an airtight container or a ziplock bag. Frozen cookies will last up to two months.