1. Line two large baking sheets with parchment paper or silicone baking mats and set an oven rack in the upper third of the oven. Preheat the oven to 410°F (210°C).

2. In a large mixing bowl or stand mixer, beat the cold cubed butter, brown sugar, and granulated sugar together on medium speed until the mixture looks slightly fluffy and well combined, about 3 to 4 minutes.

3. Add the cold eggs one at a time, mixing well after each addition, then blend in the vanilla extract, scraping down the bowl as needed.

4. In a separate bowl, whisk together the all-purpose flour, bread flour, Dutch-process cocoa powder, cornstarch, baking soda, and fine sea salt until evenly combined.

5. Add the dry ingredients to the butter mixture and mix on low speed just until the dough comes together and no dry pockets remain.

6. Fold in the dark chocolate chunks and toffee bits until they are evenly distributed throughout the dough.

7. Cover the bowl and refrigerate the dough for 30 to 45 minutes, until it feels firm enough to hold tall mounds when scooped.

8. Scoop large portions of dough, about 1/4 cup to 1/3 cup each, and form into rough, tall mounds, placing them on the prepared baking sheets with space between each cookie.

9. Press a few extra chocolate chunks and toffee pieces into the tops of the dough mounds for a bakery-style appearance.

10. Bake one sheet at a time for 9 to 11 minutes, until the edges look set and the tops appear dry but the centers are still soft and slightly underbaked.

11. Remove the cookies from the oven and immediately sprinkle each one with a pinch of flaky sea salt.

12. Allow the cookies to cool on the baking sheet for about 10 minutes to finish setting, then transfer them to a wire rack to cool further or serve warm.