Prepare the Baking Sheets:
Preheat your oven to 350°F (175°C). Line two large baking sheets with parchment paper or silicone baking mats and set aside. If you plan on chilling the dough for better texture, you can skip this step for now.
Mix the Dry Ingredients:
In a large mixing bowl, whisk together the flour, brown sugar, granulated sugar, baking soda, cornstarch, salt, and cinnamon until well combined.
Combine the Wet Ingredients:
Add the melted and slightly cooled butter to the dry ingredients. Use a spatula or handheld mixer to combine until the mixture becomes crumbly.
Add the eggs and vanilla extract, and beat until the dough is well mixed and smooth.
Fold in the Mix-Ins:
Gently fold in the toffee bits, caramel squares, and chopped pecans. For a prettier presentation, you can reserve a few of the toffee bits, caramel squares, and pecans to press on top of the cookie dough balls before baking.
Chill the Dough (Optional):
For thicker and chewier cookies, cover and chill the dough in the refrigerator for at least 2 hours, or up to 24 hours. Chilling the dough allows the flour to absorb moisture, improving the cookie’s texture.
Shape the Cookies:
Roll two heaping tablespoons of cookie dough into a ball and place them on the prepared baking sheet. Leave about 2 inches of space between each cookie for spreading. If you reserved extra toppings, press them onto the top of the cookie dough balls now.
Bake:
Bake the cookies in the preheated oven for about 12 minutes, or until the edges are set and golden brown. The centers may still look slightly soft, but they will firm up as they cool.
Add Sea Salt and Cool:
Immediately after removing the cookies from the oven, sprinkle them with flaky sea salt, if desired.
Let the cookies cool on the baking sheet for 10 minutes to set before transferring them to a wire rack to cool completely.