Remove from the heat and carefully stir in the vanilla extract. The mixture may bubble.

Immediately pour the hot toffee onto the prepared baking sheet and spread into an even layer.

Allow the toffee to cool completely until firm. Break or chop into small pieces. Measure 140 g for the cookie dough and reserve any remaining toffee for another use.

Place the butter in a light-colored saucepan over medium heat. Cook, stirring frequently, until the butter melts, foams, and develops golden-brown milk solids with a nutty aroma.

Remove from the heat and carefully stir in the cold water. Let the browned butter cool for approximately 15–20 minutes.

In a large mixing bowl, combine the browned butter, brown sugar, and granulated sugar. Mix until well combined.

Add the egg, egg yolk, and vanilla extract. Mix until smooth.

In a separate bowl, whisk together the all-purpose flour, cornstarch, baking soda, baking powder, and salt.

Add the dry ingredients to the wet ingredients and mix just until combined. Avoid overmixing.

Gently fold in the chocolate chunks and chopped homemade toffee until evenly distributed.

Cover the dough and refrigerate for at least 3 hours, preferably overnight.

Preheat the oven to 350°F (175°C). Line baking sheets with parchment paper.

Portion the chilled dough into balls weighing approximately 55–60 g each. Arrange them on the prepared baking sheets, leaving at least 3 inches between cookies to allow for spreading.

Bake for approximately 12–15 minutes, or until the edges are golden brown and set while the centers still look slightly soft.

Remove from the oven and immediately sprinkle with flaky sea salt.

For perfectly round cookies, place a round cookie cutter slightly larger than each cookie around it and gently move it in a circular motion while the cookies are still hot.

Let the cookies cool on the baking sheet for 5–10 minutes before transferring them to a wire rack to cool completely.