In a medium saucepan over medium heat, melt ¼ cup unsalted butter. Stir in ⅓ cup light brown sugar, mixing well until dissolved, about 2 minutes. Gradually add in ½ cup heavy cream, continuing to stir until thickened slightly—about 5 minutes. Remove from heat and let cool for approximately 10 minutes.

In a large mixing bowl, cream together ¾ cup unsalted butter, ¾ cup light brown sugar, and ¼ cup granulated sugar using an electric mixer. Beat in 1 large egg and 1 tsp of pure vanilla extract until smooth. Gradually add your dry ingredients: 2 cups all-purpose flour, 1 tbsp cornstarch, 1 tsp baking soda, and ½ tsp fine sea salt. Mix until just combined.

Once your salted caramel is slightly cooled, gently fold it into the cookie dough using a spatula, creating a lovely swirl effect without fully incorporating.

Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper. Drop rounded balls of dough onto the prepared sheets, spacing them about 2 inches apart. Lightly sprinkle extra flaky sea salt on top of each cookie. Bake for 10 to 12 minutes, or until edges are lightly golden.

Remove the cookies from the oven and allow to cool on the baking sheets for about 5 minutes, then transfer to wire racks to cool completely.