Preheat the oven to 350°F and grease a 13”x9” pan with non-stick cooking spray.
In a medium bowl, whisk together the flour, cornstarch, baking soda, and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium speed until smooth and creamy. Add the brown sugar and mix on medium until light and fluffy. Use a rubber spatula to scrape the bowl. Add the egg, yolk, and vanilla and mix until fully incorporated.
Add half of the flour mixture and mix on low just until the powdery texture of the flour disappears. Immediately add the rest of the flour mixture and mix until combined. Scrape the bowl and mix a final 15 seconds.
Remove the bowl from the mixer and use a rubber spatula to fold in 1 3/4 cups of your favorite chopped candy.
Use a rubber spatula to spread the dough in the lined pan in an even layer. Sprinkle with flaky sea salt and top with the reserved 1/4 cup candy.
Bake for 25 to 30 minutes, until the edges are golden brown and the center is lightly golden brown.
Remove from the oven and let cool on a wire rack for 15 minutes. Slice into squares and serve!