Line a baking sheet with parchment paper.

In a small pot, whisk together 1/2 cup unsalted butter, light brown sugar, and salt over medium heat.

Cook, stirring constantly, until the mixture thickens (about 10 minutes) and has a consistency like melted peanut butter.

Pour the mixture onto the prepared pan, letting it cool and harden for about 20 minutes.

Once cooled and hardened, use a heavy object to crack it into small pieces. Store in an airtight container for up to 1 week.

In a stand mixer (or using a hand mixer and a large bowl), beat 3/4 cup unsalted butter, both granulated and brown sugars, and vanilla until pale and fluffy (3-5 minutes).

Add in egg yolks and mix on low until just combined.

Add in cornstarch, flour, baking powder, baking soda, and salt. Mix just until the dough comes together.

Use a spatula to fold in 3/4 cup of chopped chocolate and 1/2 cup of the toffee.

Scoop the cookie dough using about 2 tbsp of dough per ball (making 18). Cover with plastic wrap and chill in the refrigerator for at least 2 hours, but up to 24 hours for the best flavor.

When ready to bake, bring the dough out to soften a bit and preheat the oven to 350°F. Line a baking sheet with parchment paper.

Place 4-6 dough balls about 2 inches apart on the prepared sheet. Before baking, top with additional chocolate and toffee.

Bake for 9-10 minutes, until the edges are just set and golden brown. Don't overbake!

Remove from the oven and let cool on the baking sheets for 3 minutes before transferring to a cookie rack to cool completely.