Step 1

Arrange a rack in the middle of the oven and heat the oven to 350°F. Line an 8x8-inch baking dish with parchment paper, leaving a 2-inch overhang on two opposite sides like a sling.

Step 2

Cut 8 tablespoons (1 stick) unsalted butter into 1/2-inch cubes and melt in a small saucepan over medium heat. Continue cooking, stirring constantly while scraping the bottom of the pot with a rubber spatula to release any browned bits, until the butter is light brown, smells nutty, and no longer foamy, 3 to 4 minutes. Immediately scrape into a large heatproof bowl and let cool for 15 minutes. Meanwhile, coarsely chop 2 (1.4-ounce) chocolate-coated toffee bars into 1/2-inch pieces (about 3/4 cup).

Step 3

Add 1 cup packed brown sugar, 1 large egg, 2 tablespoons water, and 1 tablespoon vanilla extract to the browned butter and whisk until smooth and glossy. Add 1 1/4 cups all-purpose flour, 1/2 teaspoon baking powder, and 1/2 teaspoon kosher salt. Stir with a rubber spatula until a thick batter forms. Reserve 2 tablespoons of the chopped toffee for the top, then fold the remaining into the batter.

Step 4

Transfer the batter into the prepared baking pan and smooth into an even layer. Scatter the reserved toffee over the top, then with sprinkle 1 tablespoon granulated sugar if desired.

Step 5

Bake until the edges are browned and pulling away from the pan, and a tester inserted in the center comes out with just a few moist crumbs, 18 to 23 minutes. Sprinkle with flaky salt and let cool in the pan at least 20 minutes. Cut into 16 squares.