Prep: Preheat the oven to 350°F. Line a baking sheet with parchment paper and set aside.
Dry mixture: In a small mixing bowl, stir together the flour, cornstarch, baking powder, baking soda, and salt. Set aside.
Wet mixture: In a large mixing bowl, add the melted butter, granulated sugar, and brown sugar. Use a whisk or hand mixer to mix well. Add the egg and vanilla extract and mix until smooth.
Combine: Add the dry mixture to the wet mixture and fold together with a spatula (or continue to mix with a hand mixer) until just barely combined with some flour patches left. Add in the dark chocolate and toffee pieces you made earlier. Fold together until incorporated and just combined.
Scoop cookie dough: Use a 4-tablespoon cookie scoop to scoop the cookie dough and release them onto your lined baking sheet, leaving about 2" in between each cookie for spreading. Press a few extra pieces of chocolate and toffee on top of each cookie dough ball.
Bake: Bake cookies for about 11-12 minutes. In the last 2 minutes of baking, you can press a few more pieces of chocolate and toffee on top of each cookie for a nicer look. Immediately after taking them out of the oven, use a round cookie cutter or small bowl that's slightly larger than a cookie and swirl each cookie around inside it to push the sides in for perfectly round cookies. Finish each cookie off with a sprinkle of flaky salt.
Cool: Let the cookies cool on the baking sheet for 10-15 minutes as they will still be quite soft before transferring them to a wire rack to cool completely.