Preheat the oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, leaving some overhang on the long sides to create handles. Lightly grease the parchment with nonstick spray or butter.

In a medium bowl, whisk together the flour, baking powder, baking soda, and fine sea salt until well combined. Set aside.

In a large microwave-safe bowl, melt the butter in 20–30 second bursts, stirring between each, until just melted. Let it cool for 5–10 minutes until warm but not hot.

Add the packed brown sugar to the melted butter and whisk until the mixture looks smooth and glossy, about 1–2 minutes.

Whisk in the eggs one at a time, mixing well after each addition. Whisk in the vanilla extract. The batter should look thick and smooth, similar to caramel.

Switch to a spatula or wooden spoon. Gently fold the dry ingredient mixture into the wet mixture just until no streaks of flour remain. Do not overmix; the batter will be thick.

Fold in the toffee bits, semi-sweet chocolate chips, and chopped nuts (if using) until evenly distributed throughout the batter.

Scrape the batter into the prepared 9×13-inch pan. Use an offset spatula or the back of a spoon to spread it into an even layer, pushing it into the corners. If desired, sprinkle extra toffee bits or mini chocolate chips on top.

Bake on the center rack for 22–28 minutes, or until the edges are golden brown and a toothpick inserted in the center comes out with a few moist crumbs but no wet batter. Slight underbaking keeps the blondies chewy; they will continue to set as they cool.

Place the pan on a wire rack and let the blondies cool completely in the pan. If desired, sprinkle with flaky sea salt while still warm. Once cool, lift the slab out using the parchment handles and cut into 24 bars (or smaller pieces for parties).